Here’s my personal suggestion for each banner
Q&A - Scizor (because it’s Pokémon database’s mascot, and Q&A is the most used community.)
RMT - Incineroar (because he reflects the fiery compassion of RMT, and nobody uses him, just like nobody uses RMT.)
Meta - Metagross (Not only does it have Meta in it’s name, it also is a steel type, which fits with the color scheme and mood of RMT. Also, he’s gray, just like the color of Meta’s banner.
Chat (?) - Idk if this counts as a forum but if so, Chatot because it has Chat in the name and is known for chatting a lot. I also think that Chatot really needs more love as it’s generally outclassed by other Normal/Flying types like Staraptor, and I thought seeing it as a mascot for Chat would be nice.