Well I had two ideas I'm just gonna put them both in this thread.

Capture the flag.
People choose a kit they want before the game starts and is put on a team. Blue or red (or whatever colors would be picked) Maybe people with a rank get something.
Something like infection did: kits give you exp and you level up. Each level has a title and a perk to come with it.
Non ranked players could vote to get exp bottles. (2 or 3 per vote link) The goal is get to the other teams base to grab the flag (banner) and return it to your base to win the game.

Murder Mystery: Prison:
Prison was a fun gamemode but we could add a fun twist to it maybe? Murder mystery in a prison (not a big prison.) I never seen a map in MM (murder mystery before) I play murder mystery on other games gmod etc.
10-15 players per game.
All given a role: Innocent, detective or murderer. Cells could have hiding spots or could hide weapons (bow and arrow) for innocents to find and use to defend themselves with.
Detective could get a reloaded bow that reloads after a few seconds after shooting.
Murderers goal is to kill every player before a timer runs out or they are killed by detective or a innocent by stander. Murderer role gives a iron sword which would be used to kill other players.
Rank players could have different knife options. Example: VIP - cooked chicken (idk something goofy)
When you vote you could earn exp bottles to level up and possibility to earn different titles.

Idk just some random ideas. In a nutshell we do need more game modes honestly.