From what I can gather, the frontrunners go something like this:
Kiryu-The easy path. Massive in Japan, rapidly gaining in the West, flagship series for Sega. However, he's also A: From a mature series, and B: not usually on Nintendo systems, making him somewhat of a Joker situation but with more legacy.
Arle-A more Nintendo-y alternative to Kiryu, so to speak. Is big in Japan, is gaining in the West, with the added bonus of being fairly friendly towards Nintendo systems. I also hear she's a popular request in Japan, but I'll have to double-check.
Sakura-The Japan-appeal character. Absolutely everywhere in Japan (Anime, novels, stage shows, etc.), the series ranked the highest in both a "Franchise revival" poll and a general "Franchise investment" poll
at it's low point, and is generally beloved by the Japanese. In the West? ...Not so much. I mean, the series is at least making an effort nowadays, but it's still a pretty big barricade. It all depends on just how focused on Japan Nintendo wants a character to be.
But hey, at least she's in a better spot than someone like Takamaru
I
think Axel could be added to the list as a West-appeal character, but I don't know enough to say, so yeah...