Here's an idea for a fun round. A wildcard fun round. I haven't fully flesh this in terms of gameplay (whether it's a normal ttt round, tdm/dm, etc.) out so this is open to suggestion. However, the gun you have out will have random stats. For one instance, it can have fire rate of a huge and damage of a HK SL8. In another instance, it would have fire rate of p228 and damage of a glock.
This thread is rather old, but something like that has been brought up before: https://www.seriousgmod.com/threads/fun-round-that-has-randomized-weapon-stats.42665/ There might be some arguments or ideas in that thread.