Honestly I'd be quite happy to hear the case for Dark Souls, but I will warn - I'd be surprised if Bandai Namco, if asked (and I suspect they would be) decided to choose a franchise that is effectively over.
Tales Of and Digimon both have new games coming up that Bandai Namco have high hopes for. (Also in Digimon's case the Vitality Bracelet and new card game also seem like things they're pushing hard right now.)
About Souls being effectively "over", I have a couple of things to say. I think Souls isn't going to be de-facto over for a long time.
The Souls series has proven very profitable for Bandai Namco is a relatively short period of time. In only 10 years and 3 (4 if you count the remaster) games, the series has become their best selling RPG franchise, and the 6th best selling JRPG franchise worldwide. Dark Souls III remains their fastest selling game to date. Namco also seems to have taken notice; the Solaire Amiibo is still their only one. They chose to go through pretty significant expense to make their only Amiibo figure for Dark Souls, instead of any of their other franchises (most notably Taiko and Digimon, since they are most often associated with physical merchandise). What's more, Dark Souls is one of the only two series to have an Amiibo, but absolutely no representation in Smash yet.
Sorry for how many times I've said this here, lol.
Despite the Amiibo releasing during Smash's development cycle, and being made by the company which co-develops Ultimate, not even a compatible PNG has been added to Smash yet. Maybe it's Bamco being humble, but I don't think that licensing a spirit for Smash was impossible if the companies were willing to make an entire Amiibo. Most other Amiibo which had no series representation in Smash have been slowly made compatible post-launch. Also, the Solaire Amiibo is super useless at the moment. It's only designated purpose in one game; Dark Souls Remastered, and all it does is give you a cosmetic emote which is obtainable through normal play.
Only 1/3 of the mainline games have been ported to the Switch as of yet, and both the second and third could run on the Switch (and definitely the Switch pro) with some minor adjustments. Though Elden Ring and future Souls-like FromSoftware games might lack the title of "Souls", they are still very much seen as part of the same family of games.