3DS sales aren't especially great in the markets that Western developers like Ubisoft and EA care about.  Even moreso when you look at software.  At the same point in its life the DS was selling more software in The Americas and Other in one fiscal year than the 3DS has done in four.  The situation isn't as extreme when compared to the GBA, but even there the GBA had sold more than double the software the 3DS has done in the Americas.  The 3DS does stack up much better in Japan and Other.

It really makes me wonder if Nintendo is truly going for a unified approach where all or most games are playable on either the handheld or console then will the likes of Electronic Arts, Ubisoft, Activision port their games to it?

Wii U doesnt have the install base so these publishers don't feel the need to port over games for 200-300k or so sales and while 3DS does have a decent install base (over 30 million in the west) it doesn't have the specs to handle ports so it can only run cheap spinoffs and these publishers don't want to build games from the ground up for 200-300k or so sales.

So in theory if NX handheld/console are able to share games and able to run ports of 3rd party games, would 3rd parties release them if they were able to muster up 500k-1 million sales for ports of games like Call of Duty, Assassin's Creed, Madden, FIFA?

If it was actually able to push as much as a million I'd imagine those ports would happen.  Especially for the sports games.  I don't think just lumping in the handheld install base would guarantee that though.  It's more of a demographics issue than an install base one.  FIFA 14 on PS4 sold more in Europe on the console's launch week than  FIFA 15 has sold on the 3DS worldwide lifetime despite the huge install base differences there.  FIFA 15 PS4 week one in Europe sold more than all four FIFAs worldwide lifetime combined on 3DS.

It'll also depend on how they actually handle a unified approach.  How powerful would it be, and would there be any differences when playing it handheld versus hooked up to a tv?

maybe, but the interest in the nintendo hardware is not strong or good. i think.

with awesome sales (next hardware), maybe ubisoft coming back. but EA... ? well..

Ha! The Shit list!

I think both Ubi Soft and EA will make some games for the NX so long as it sells better than the Wii U and GameCube and isn't cripplingly scaled back in its hardware.

As I've said Japanese developers that are more hard on Nintendo, Square-Enix has given Nintendo one notable console release in the last like 20 years ... and that was Dragon Quest X, a game that had little/no benefit to Nintendo because they opted to make it a MMORPG (nice going Square-Enix).

Good riddance, easily my two least favorite game developers