There are basically two big things that I think make Morrigan possible, albeit somewhat unlikely:
1. We seem to get a traditional fighting game character during each wave of Smash DLC (
for Smash for,
for FP1,
for FP2).
2. Our fighting game characters have, thus far, come exclusively from the golden age of Japanese arcade fighting games during the '90s.
This alone gives Morrigan a niche she could occupy. Darkstalkers is still a fondly remembered and influential classic of that era, especially in Japan. Being Capcom's other big fighting game franchise during that era certainly helps. Plus, Morrigan has a pretty unique moveset in terms of normals and potential mechanics that could be introduced and... other qualities that continue to make her incredibly popular 20+ years after Vampire Hunter/Saviour 2 was released.
That being said, Chun-Li, Sol, Scorpion, Nightmare, Nakoruru, a (dedicated) KoF character, and Kasumi could all theoretically occupy this spot as well, so I definitely agree she's not a shoe-in. Her being in competition with Capcom's stable is also a roadblock, but I have a hunch we'll get more than one Capcom character next game. There are just too many great choices to not double-dip.