Lets create a list of all the custom gamemodes users have created on Overwatch.

I can think of a few, but I want to play more that I haven't yet. I'll try to go from most popular to not really known / ones only me and my friends have created.

Ana paintball - Basically it's 6v6 ana, and you only have your primary and your sleep darts. You just try and eliminate the enemy team. Usually one shot or one dart is an instant kill.

Attack on titan / monster hunt - This can vary but usually it's 2v6 or 3v6, the smaller team are the "titans" (Reinhardt, roadhog, winston, etc) and the bigger team are the ones fighting them (tracer, lucio, etc). Titans have huge amounts of hp and can one shot them, but the fighters are super fast and have really low cooldowns. Somehow the game is pretty balanced if you get the settings right.

Don't move - This is where it is 2v6, and the 6 players have to try and get to the other side without being seen moving by the 2 players. The only rule is you have to get to the other side without being seen moving. Some obvious things such as Sombra invis are disabled. This kind of relies on the 2 people being cool with it, because once one person gets to the end they have to suicide.

Hide and seek - Pretty much self explanatory. Go on a really big map, and 1-2 people try to find where you are. If they do, you're dead.

All melee only gamemodes - Basically it's 6v6 and you can only melee / torb hammer and it one shots the enemy team.

Reinhardt Joust - 6v6 reinhardts, no damage. Only on maps where you can charge off the map (Charge cd 0%). Speed either normal or set to max. You just charge over and over and try to charge them off the map.

All "schools" - Lucio wall ride school, Junkrat double jump school, roadhog hook glitch school, sombra skybox school, etc. Anything that requires practice and knowledge of maps.

Lucio boop - Pretty much similar to the Reinhardt Joust, except it's Lucio right clicks now. This is a lot fast to win with, because it's easier than trying to joust at 500% speed.

Widow headshot - Basically just 6v6 widow, headshots only.

All the other RP gamemodes - These can either be really good or really bad. You just need people who aren't going to troll.

Finally, our most played / proudest gamemode amongst my friends:

Auschwitz RP. Or if you're offended, Bill Cosby RP. If you're still offended, Sleep Dart RP. - Basically it's 6v6 with all anas. Everything is disabled except her sleep dart (and you can turn on the ults too). The sleep dart cd is set to 0%. You fight and you keep sleeping the enemy team to keep them down on the ground. This might seem like it's boring, but once you start to get some fun and creative people, it becomes really fun. You can have traitors who let the enemies wake up and go kill the other team. You can have truces and truce areas / truce emotes. You can have backstabbers who say they will truce and then go against you, and then fall down again. There's really no real goal, but the same 12 people played this for over 2 hours so you can really get into it.

So those are basically the main ones I can remember / have played. If you guys don't see a gamemode you know that's not up here, please post it so that I can try it with my friends!

User submitted gamemodes I have not known about

Mercy Paintball - I'm assuming this is just like Ana Paintball except it one shots with mercy secondary. Seems pretty fun. You also turn on res so you keep reviving your teammates.