I seem to experiment with house rules in every campaign I run.
Sometimes I do max HP for first level, crits & fumbles, and sometimes I don't.
Typically in any OSR game I run I do group initiative d6 higher wins, ties go to players. I've been experimenting where I just roll 1-3 players go first, 4-6 monsters go first. Anything a bit more crunchy where it seems individual initiative will have more of a noticeable effect, like 5e, I just run it as is.
Ascending AC is the only thing I can think of that I almost always use, and usually pick a ruleset or clone that already incorporates it.
I'll work with players with alternate classes or abilities, but I don't bring them to the table myself.
I guess my houserules are never perfect and I'm always tweaking them.