By which I mean stuff like Hackmons, Tiershift, STABmons, and CAP.
Despite the fact that these do not use official Pokemon (at least not in legit manners) they are accepted metagames that are as official as the tiers. They even have a specific metagame option for them on Pokemon Showdown. They have their own strategies and threats that you need to deal with, and it's not just a matter of suggesting more broken Pokemon.
I feel no downside in letting these metagame types on the RMT. By having them, we can allow people to increase their knowledge about these metagames and create better teams for them.