if MS gets a massive RPG, it'll be a new IP that has the potential to work well in the US.
SE will never put their proven big guns solely on 360, but they might develop a new one on there (here's betting on Infinite Undiscovery and SO4 to be recieved well enough to prompt a massive game from square).

 New? Look at the 4 behind the SO. 

If something is better than FFXIII quality-wise, then that might be either FFXIII Versus or something from Tri-Ace, Level 5.

Microsoft made lots of compromises and lost lots of money when they entered the console bussiness. If they want to offer the widest variety of RPGs and be the console of choice for RPGs in the upcoming generation, they could invest in making DQX exclusive.

Final Fantasy is the second biggest RPG franchise today, only behind World of Warcraft.

Will 360 ever get an RPG as popular as FFXIII? No.

Will 360 ever get an RPG as good as FFXIII? Almost definitely yes.

you missread me : new is for "enough to prompt a massive game from square" as in they'll create one if the market is really there.

I don't think IU will be on that scale but it's a first step 

To get anything as popular as FFXIII on a console is next to impossible.

The biggest titles are these:

Pokemon - will probably never be on a home console for real.
Dragon Quest - could happen if the 360 sees some good sales in Japan. Not impossible, but next to.
Final Fantasy - The 360 could see a spin-off FF, but probably no main part of it.

Now, for possibilities.

Starcraft - some would say Starcraft is a bit RPG ish, and letting this onto the 360 is the most logical for consoles. SC2 has pretty low SRs, so it could be on the 360.
Final Fantasy 13 port. If FF13 fails on Ps3, it coyld be ported to the 360.

its a dead end question really because if FF did come to the 360, it would cease being 'as big'. Gamers are not buying 360's to play J-RPG and pumping money into 'forcing' the genre onto it isnt going to work.

MS could buy SE, providing they wanted to be bought (which they don't) but even then the cost would be astronomical. They would need to cover the potential losses that SE would make from no longer supporting the PS3, PSP, DS and Wii, which considering those consoles take up the vast majority of the market, would be an insane amount of money, even if they were left to publish on handhelds.

If MS knew that they could just scoop up Japan and the entire RPG playing market in one purchase like that, they would but its an incredible gamble and one that wouldn't be worth the risk.

MS are clambering over themselves to buy up smaller RPG series but they have their head up their ass if they think that's going to make a huge difference in Japan. The Japaese market love RPGs, but they also love Gundam, Dating sims, Train driving sims and endless other bizarre games that the west just will not touch and as long as japan keeps buying the PS3 over the 360 for those games AND RPGs, they have no chance.

Lost Odyssey - The game which was considered to be rival of Final Fantasy, from Final Fantasy creator SOLD only 700k.

Final Fantasy selling atleast 500k. And not just because of name, but because of High Quality.

FFXIII, if it would be x360 exlcusive, would sell maximum 2 mln., Jrpgs are NOT popular on x360.