In my opinion, it's really not worth thinking about
if they make echoes across series for hypothetical characters when we don't know
who those characters would even be to begin with.
Most of Smash characters come with character specific gimmicks that only work for themselves and potentially other characters within the same franchise. The only exceptions would possibly be the Marth-lings which use their 2 types of swordsmanship and Captain Falcon who's really just a full on brawler with fire effects.
The sliver of space in the venn diagram between "characters that aren't part of the same series" and "characters which are a feasible echo candidate" is so infinitesimally small I have to wonder what the point is in discussing it, and with the quantity of feasible candidates being so small, if Sakurai would even consider it in the future considering historically they've only ever been made to compromise fan appeal and limited resources.
... But sure, give me Raidou Kuzunoha as a Joker echo!