Characters get cut because of time restraints or technical issues. It’s not because they “aren’t relevant”.
Now, if we’re talking about priority then there’s a hierarchy to that as well. If you’re a 3rd party property that has a contract, you’re automatically in. They aren’t going to form a contract and then go back on it. Snake is a different case, as I imagine Brawl was a one time thing unfortunately.
So, if all 3rd parties are planned to return, they will probably all return.
As far as cuts to the first party characters, that could be because of workload, longevity to Smash, diversity in moveset, etc. Unique characters probably won’t be on the chopping block if they’re very different from others because that’s literally the point of their inclusion. Duck Hunt, for example, plays nothing like Marth. They not only have different movesets, but you can’t play them the same way.
You could even say Roy and Lucina, despite being similar to Marth, can’t be played the same either.
Anyway, once characters get put into Smash, they basically become a part of the “family”. They don’t get cut because they have a target on their back, they get cut because they have to.
Time restraints or tech issues.