I think TF2 would be heavily improved by re-implementing team switching, auto-scramble/vote-scramble, much longer round timers, etc.
Community servers run these features more often than not and they're viewed as near ubiquitous benefits, I think it's worth considering.
Thanks Valve for taking steps to unmute F2Ps in TF2!
Many players would like to see you look into changing Matchmaking next. Casual frustrated us in 2016, and frustrates us still.
If TF2 is to be let off into the sunset, it would be great to see it reworked and/or reverted.