What I mean is that Illumi is the catalyst of a much, much more important overarching subplot which is yet to be resolved. This Troupe stuff while cool as fuck is still essentially supporting characters trying to kill a main(ish) character because that character is on a salty bloodpath. Illumi has relevance in the series far beyond such trivial matters with Hisoka.
Add in tid-bits like Nanika being from The Dark Continent, Gotoh's death being unknown to Killua, Zeno and Silva not seeming that bothered that Killua removed the needle that whole crazy mountain still convinced that Killua will eventually return as an assassin - obviously there's going to be arc in the longrun, there's just so much to cover still and there's no way Illumi's not going to be around for it.