I think Atlus has expressed interest in bringing Mother 3 over, but as I said on another forum, Nintendo is oddly protective of their properties. Atlus also wished to bring over Densetsu No Stafy as the story goes, and Nintendo blocked them - despite having no logical reason to (Stafy contains no content that would be objectionable to people in the states - no more than Kirby, anyway). Infact, as I recall, the story about Atlus attempting to localize Stafy came from a Starmen.net staffer when in regards to them speaking to an Atlus rep over localizing Mother 3 for Nintendo. If either game is coming over, it's going to be localized internally by Nintendo themselves and nobody else.
Mastiff, XSeed, UFO, NISA are great, but none seem to carry the weight Atlus does (infact, outside of Mastiff, I've never heard of those other three; unless NISA is Nippon Icchi). If Sega or Sonicteam is going to trust anyone, it's going to be Atlus simply for their experience. I'm sure it's all a matter of perspective, but it's clear to me why something like Tales of Fandom stayed in Japan. Ys had it's shot at the American market already and apparently wasn't worthy of sticking around (I recall reviews being pretty mixed). I've never played a Ys game myself, so I probably don't "get it", though.
But NiGHTS... the PS2 remastering is all anybody would've ever needed. Nobody in their right mind will claim Journey of Dreams surpasses it's predecessor (and saying this full well makes me think the same could apply to Ys above), and considering the demand for more NiGHTS pre-Journey, I'd say the PS2 remastering out-prioritizes most of the other "this game must be localized immediately!" options. And don't forget - it helps to strike while the iron is still hot.
Even better, it's not as though NiGHTS is a terribly text-heavy game to begin with; it's not like they're localizing Odin Sphere or Persona 3. Chances are all they'll have to do is tick the "USA Region" checkbox, retranslate the manual, revoice the Christmas intro, and they're more or less done. That's what you could call easy money. The only thing stopping me personally from importing it is the requirement of a modchip (and the money to fund both the purchase of a means to play imported software and the software itself, but that's beside the point).