Will there be any full clear ILs or lobby categories added?

Not likely at all. They generally want to make sure that the boards won't stay empty forever which would probably be the case with full clears. I recommend talking to the moderators directly tho. Iirc they refused to make a beginner lobby arb category for sc2020 despite someone having done a run. Might be different this time around who knows.

about full clears, creating a leaderboard for a category such as arb could.motivate people to run it

i gave up on begginer sc2020 arb when i saw there was no leaderboard

I used the same line of argument but the consensus seems to be that not boards should be first but interest in running it. Id just make boards for every category including all lobbies ngl. Dont think its really a shame if a category stays empty.

I am currently planning to get all of the fc silvers through expert (after I'm done with 202) and make them verifiable, since I would assume that a big reason to not add fc to the leaderboards is because they wouldn't have runs would this change the possibility of adding them?

Ask the mods directly over discord if they are willing to add the categories after you have done the runs.