I'm sad that I'm the only vote for custom game modes so far
I have the most fun doing them because tf2 is a ten year old game and people often become too focused on a meta, and I want them to learn new ways to play the game. All of the maps I've ever submitted to the site are takes on fresh new modes that hadn't been done before. This includes medieval player destruction, melee only class restricted pass time, or playing basketball with your own body.
It doesn't take an incredibly expensive knowledge of logic to know how to do them, either. You can flowchart almost everything you want done for a mode and then research how to do each individual part. Learning how to do the logic is also the most fun part! Whether it be a simple gamemode where you have a combination of medieval and pass time or a bumper kart arena where there is 0 documentation for any of the entities, you will learn a lot about how the game works and how to solve problems in the future. Even if the engine feels like a lot of things are hard coded, you can usually re create almost anything you need. HUD elements can sometimes be limiting but you can accomplish a lot with the robot destruction or play destruction huds. GD_rodense is a good example.
Testing these maps can be hard though. People in a playtest usually don't know what they're in for, and play to test more traditional maps. Sometimes people will be upset by you limiting their class, restricted them to melee, or making them try something they are unfamiliar with. But remember they are there to test, so I put it all behind me. More valuable feedback can still be hard to get when testers don't want to like what they're playing.
If you wanted to start making something custom, a good way to see what's possible and what isn't is to browse the insanely expansive list of player conditions and attributes. Anything a weapon can change, your map can change. You can create tf2 zombies or replicate the saxton hale mode, it's all possible in vanilla!
I definitely still recommend people to map for a new gamemode, though, and not to be afraid to push the limits of the meta of mapmaking! Without it, we wouldn't have watergate, probed, byre, or even pit of death. I really like to see people learn new ways to play a 10 year old game.