I personally feel difficulty is a major factor in how popular a guest character is, like how Mai is super easy in DOA and everybody and their mother uses her since she's easy to pick up and win with on lower level and higher level since people still dont seem to know how to deal with her.Character difficulty was never a reason for cast issues. It's what I always say, there are bad guests and there are good ones. Guests in general are an upgrade to the game even if it's guests nobody wanted. It could be one of the shittiest guest characters and it can still benefit the company because of the partnership one can do. I think Noctis was a terrible decision by Namco for T7 (I still don't get why people are hype for this guy in the game, he's not even a fighting game character). Even if he was made from scratch, he still looks damn weird. Maybe because that gameplay of his just doesn't fit the atmosphere of Tekken whereas Akuma and Geese fit perfectly in that environment.
It is true what Wazaaa said though. They generate more hype and just a better overall result even if it was a shitty inclusion from a shitty game. The worst guests are the console exclusive guests. Those get banned and just a waste of damn money. Why bother learning a console exclusive character when it's going to get banned anyway lol.
Geese and Noctis were ok but at this point I think legacy anything is more wanted than guests since Geese and Noctis aren't really talked about much besides maybe combos and such although that was different on day one when everyone was excited to use both the characters