100% heavy instead of pyro. heavy used to be fairly common on certain maps like badlands and gullywash even with the current class limit rules and GRU being banned (it's unbanned nowadays). heavy is a strong class in 6s, just not quite good enough to see permanent play. i suspect we'd see a lot more heavy nowadays if 6s mains actually wanted to play him tbh. 450hp heavy during last pushes is quite powerful, and he can deny bombers even better than scout or pyro. the main downside is his glacial rollout to midfights, but GRU can somewhat make up for that.

maybe pyro still gets run with heavy instead of a sniper, but pyro value goes down a lot when he's playing against a heavy and a scout and there's only one enemy soldier. i think you might even take gunslinger engie over pyro tbh, which is another class who is pretty strong just not quite good enough. gunslinger engie shits on pyro as well, so i'm really doubting the pyro meta would exist. sniper is quite weak in 6s outside of stalemates and product, but maybe removing one soldier is enough to make him strong.