@Ocean22 @morguldir @digital i've actually put a lot of time into working out how something like this would work, because i want one too.
an important first step (imo) is to get OpenID Connect support into Pleroma. https://openid.net/developers/uncertified/
this will let you leave your instance ui while still carrying your identity with you. this opens up a lot of options, specifically: topical forum servers.