Game Modes

Co-Op
Player 1 and Player 2 would each be assigned a color. On each floor, there would be two item rooms and two shops. The gold trim around one of the item rooms would be Player 1's color. The gold trim around the other item room would be Player 2's color. The two shop doors would be tinted different colors as well. Keys would be the different player's colors as well. The keys can only open doors with their matching colors. Also, Player 1 can only take the item from their item room/shop, and Player 2 can only take the item from their item room/shop. Keys, bombs, and coins can be picked up by anybody, but they can be dropped to give to the other player. Any regular boss would get destroyed by 2 people, so new bosses and floors would be added to the game. An example of this would be "Uberchub," a superpowered version of Chub. It would have the ability to target both players, have more health, attacks, and deal more damage. The way death would work is if one person dies, they are dead for the rest of the floor. If both players die, the game is over. However, if one player dies, yet the other player manages to complete the floor, the player that died is revived. After the boss fight, the boss item that is dropped would be a revival item instead, so, essentially, there is no boss drop. An example of a new floor would be a water-themed floor with octopi and squids and mermaids.

Suggestion:

Maybe when you are revived you should be at half health or less. Just to balance it out. Or do a Left 4 Dead thing where you can only revive so many times before you truly die (and for that maybe have a really rare boss drop the revives completly dead players).

Race
The game selects a random opponent(based on a skill level and unlocks) or you can invite a freind with Steam. the two would play normally, starting on different floors. However if both players hava something like Catacombs 2, the progress (except for the boss) is the same. The map is not revealed. Items left behind or rooms unlocked by the other player are still the same. Since you can't fight the same boss for the floor, different bosses would be fought, with the exception of Mom, Mom's Heart, It Lives, Satan, Isaac, and ???.So if Player 1 fights Daddy Long Legs in Necropolis 1, Player 2 would fight Mask of Infamy or whatever else there is to find. However, if players are on different floors with the same bosses, or on the 1st level, it would be possible to fight the same boss. Such as Player 1 could fight the Duke of Flies in the Cellar 1 while Player 2 fights the Duke of Flies in Basement 1. To make it fair, both start as Isaac and if one player has unlocks the other doesn't, they aren't used. So if Player 1 has unlocked Steven, C.H.A.D, and Gish, and Player 2 only unlocked Steven, that means C.H.A.D. and Gish would not be fought. There would be a chat system to, well chat, and alert players of important notices such as:"[insert name here] is now on Cellar 2." or "[insert name here] has died on Womb 1." The goal is to complete the game as far as possible (like I said about fair unlocks, so if Player 1 has Sheol/Cathedral and Player 2 doesn't, you have to complete as far as Player 2 can" or whoever gets the furthest without dying. This also could be down with any amount of people.

Boss Rush
In this game mode, you battle every boss in the game. Each floor has a straight line of rooms with doors leading to boss rooms on the northern end. Every first level of each chapter is on a regular floor, while every second level of each chapter is on an alternate floor. Each floor has a shop, a library, a trinket room, and two treasure rooms which don't require any keys to enter.

The Basement, the Cellar, the Caves, and the Catacombs have 5 bosses each, while the Depths, Necropolis, the Womb, Utero, and Sheol have 4 bosses each, and the Cathedral have 3 bosses each. Alternate bosses are counted as bosses. Each boss except the last one you defeat in the Cathedral drops an item. The Fallen, Krampus, and Satan will drop Devil Room items, while Isaac, Triachnid, and ??? will drop Angel Room items. Mom's Heart and It Lives drop Mom-related items, including Mom's Knife, which can be dropped even if it isn't unlocked. You can battle the bosses in any order, with the exceptions of Mom, Mom's Heart, It Lives, and Satan. These four are always battled last.

After leaving the room of the last boss you defeated on that floor, you will be teleported to the "I AM ERROR room", where you will also be fully healed. Teleportation does not work, and will give you 5 cents instead. Two of the rooms have one version of a sin. For example, if Cellar 2 has Pride, it will also have Super Pride.

Hearts in shops will be replaced by health upgrades, while keys in shops will be replaced by random treasure room items. Shops will never have Greed in them; he is encountered in the regular rooms. Little CHAD, rather than dropping half hearts after completing a room, will drop two full hearts.

Completing the Boss Rush unlocks extremely powerful items that are very rare to encounter.

Basement Bosses: Cellar Bosses: Caves Bosses: Catacombs Bosses: Depths Bosses: Necropolis Bosses: Womb Bosses: Utero Bosses: Sheol Bosses: Cathedral Bosses: Fan made ideas of bosses would be in boss rush too.
 * Monstro
 * The Duke of Flies
 * Gemini
 * Larry Jr.
 * Steven
 * Widow
 * Pin
 * Blighted Ovum
 * Gurdy Jr.
 * Famine
 * Fistula
 * Gurdy
 * Peep
 * Chub
 * C.H.A.D.
 * The Carrion Queen
 * The Husk
 * The Hollow
 * The Wretched
 * Pestilence
 * Monstro II
 * Gish
 * Loki
 * War
 * Mask of Infamy
 * Daddy Long Legs
 * The Bloat
 * Mom
 * Scolex
 * Blastocyst
 * Death
 * Mom's Heart
 * Teratoma
 * Lokii
 * Conquest
 * It Lives
 * The Fallen
 * Krampus
 * Satan
 * Headless Horseman
 * Isaac
 * Triachnid

PvP
In this game mode each player must choose 5 items and they will fight each other with special items for diferent arenas like:

In the caves are:Lil' gish,Lil' chubby and Trandescence;

In the cellar:Mutant spider,Best friend and Daddy long legs

Also the players must find the items in the chalenges or the original playtrough

Holy V.S Unholy
'''A PvP in which player 1 starts out with Jesus Juice or the Halo, and player 2 starts out with Whore of Babylon or The Nail. Each player has 3 soul hearts and has to kill the other player to win. It has 3 arenas: Cathedral, Sheol, and Womb. There are no treasure or item rooms and whoever wins is labeled "The Master"'''

RPG Mode
In this mode, there is a EXP Bar that is placed in between the trinket and pill holder slots. Above that, it states what level you are. For each enemy you kill, you gain a wedge on the EXP bar. Each level has a base level on what level you should be. Below are the levels. (Also, if you are underleveled, you have a greater chance of going to the easier floors.)

Stats
Stats are a bit different in RPG mode. basic stat-increasing items (range, speed, damage, hp, tears) no longer appear, and instead you get a random number of stat points based on your level (see chart below). You then distribute them throughout the basic stats.

Items
In RPG mode, stat items no longer appear (see above). Instead, you will find passive upgrades and activated items. I would add level requirements, but ohmygoshthatwouldtakesevenyearsbecausethereislikesevenhundreditems.

EXP
EXP works like this- spawned enemies, black flies, and small red flies do not give any EXP, and bosses give 10 Exp (champion variants give 15). For bosses that appear on any floor, they give 15 amount on Basement and Cellar floors. Other then that it is the normal multiplier. For each level, it takes 5 extra EXP. The base amount needed to level up is 10. For every normal enemy killed, you get 1 EXP. For every champion version you kill, you get 3 EXP. Sins give 4 EXP, and Super sins give 6 EXP.

Harder mode
As the title says, it's a harder version of the actual game.

Difficulty increases in the following aspects:
 * From Basement 1/Cellar 1, every hit does a full heart of damage.
 * From Womb 1/Utero 1, every hit does 1.5 full hearts of damage (1 full heart if you have The Wafer).
 * Normal drops after completing a room are twice as likely.
 * Non-item chests drops 1.5x their contents.
 * Every enemy has 25% more health.
 * Every boss has 15% more health.
 * Deals with the devil/angel rooms are rarer.
 * Every deal with the devil costs half an extra red heart / one extra soul heart.
 * Angel deals require you to fight the angel (who will activate as soon as you enter) before getting your choices.
 * All normal enemies become champions.
 * Champion enemies very rarely drop hearts/keys/etc when they die.
 * Beggars need more money and give drops less frequently.
 * Devil beggars need more hearts and give drops less frequently.
 * Tinted rocks are rarer, but their drops will be the same.
 * There is a perminant 1/2 for a curse on a floor, but a 1/10 chance for 2 different curses.
 * curse of the cursed is added to the pool of curses if you aren't playing the lost/tainted lost
 * Any of the seven sins drop their items very rarely.
 * There are more possibilities of having a mini boss in normal rooms from Caves 1/Catacombs 1.
 * Poop drops are rarer.
 * Golden poops appear less often.
 * Corny/red poops appar more often.
 * Fireplace drops are rarer, and all (excluding white) fireplaces are red/purple
 * Red/purple fireplaces shoot more often and take more shoots to exsinguish.
 * Enemies that explode when they die make an explosion with a higher radius.
 * Curse rooms/secret rooms items almost never appear.
 * Shop offers less items and they are more expensive.
 * Sacrifice rooms require more hearts.
 * Slots machine pay less frequently, same with blood donation machines.
 * Libraries are less common.
 * Enemy.
 * Pills will be bad most of the tims. (PHD/Lucky Foot/False PHD only decrease the chances of pills being bad by 50%)
 * You start with less luck in addition of everything else.