It really depends. They could dynamically analyse some recent (popular) content, group it into categories and suggest users that wrote that content. This way suggested users would be up to date and not some chosen people forever, regarding if they are active etc.

Yeah, but that's the Catch-22 I mean. You have to already be popular, to have popular content. That's why follow lists have been getting more and more similar, with recommended or trending content often being the same 5 people over and over, with every post they make. Gigantic bifurcation in the number of followers (200k versus 20), is the cause, and we can't end the cycle until we figure out how to surface unpopular-but-interesting content.