I wouldn't say unpredictable. We can form a reasonable idea of what they'll do based on precedent. Of course, we won't get it right every time, but we can definitely narrow things down.You know, I use to be so sure Crash would be coming as one of the last newcomers for Ultimate. But now, I'm not so sure anymore since no one is truly a lock. I have had some predictions thrown out here and there, but I have no real idea how Ultimate's DLC lineup is going to end as Sakurai and co are so unpredictable.
With first party DLC, Nintendo leans HEAVILY towards promoting recent games. With 3rd party in general Nintendo almost exclusively has added characters from franchises with very high sale numbers and/or that are well established/classic. Literally the only exception to this has been Bayonetta, and she's practically a Nintendo character.