Nintendo is clearly interested in full collaborations with japanese franchises. I repeat it various times but 9 out of 11 best selling third party IPs have full representation (character, stage, music, spirits and AT/mii costumes) in smash ultimate. The only ones left are Resident Evil (only spirits) and Monster Hunter (that has everything besides a fighter and a multiplayer stage). Both are huge successfull, MH World has sold more than 20 million copies (thats Nintendo territory) and RE series as a whole has sold 120 millions, third best behind Sonic and Final Fantsy.
So yeah, the next move for Nintendo would be negociate with Capcom to get more RE and MH content. At least if MH developers don’t want the hunter itself, they can get another AT, a multiplayer stage and much more music and spirits.
Other popular japanese IP that may cater Nintendo sight is Dark Souls. The third game sold more than 10 millions and its best selling Bandai-Namco game this gen. Although it cointains Souls name, very soon Bandai-Namco will published next From Software game, Elder Ring (thats basically a Dark Souls world game). It can happen that we get something from Souls next time, at least an AT or mii costume, or maybe a full collaboration with a fighter.
Of course, more small characters, like Terry, can still happen. Everything don’t have to be massive names.