There are certain things about X1 that really only work because it is X1. The armor upgrades in particular, outside of the buster they're all either not applicable or over-used by now. The character designs are considerably more simplistic than the series in general, and the sprite style really tailored towards such (which is why I maintain that reverting to the SNES sprite style is a poor choice).
What X1 did right, which I wouldn't mind seeing revisited:
- Ass-kicking special weapons.
- Boss weapon effects outside of the usual weakness (incidentally we saw some of this in Xtreme2 with Ray Claw).
- Buster upgrade of at least triple strength, plus
frikkin' shockwave (DAMN YOU, MHX!!! 
).
- Helpful stage-clearing bonuses. Frozen lava, flooded jungle, and such.
Really, though, if MM9 made one thing clear to me it's that returning to roots and ignoring the advancements made are two different things. I will never stop cursing the fact that Rock refuses to slide. There is no excuse for that not at least appearing as an option or unlockable (WITHOUT double damage/knockback, thank you). It's also hard to wrap my mind around the fact that some idiot still finds the MM2 NES weapon-drum appealing enough to copy-paste. For all the audio manifestations of greatness in NES MegaMan, that was not one of them.
Returning to roots does not mandate reverting the visuals to an early stage, either (NSMB and Metroid Zero Mission come to mind). And the X-series is, I feel, among the worst possible candidates for such an approach. The X-series sprite style is the textbook definition of a lack of foresight. They redesign Zero, who at this point is a long standing co-star, and the corner that they drew themselves into becomes painfully inadequate. There is no reason to return to that.
And if the Xtremes, 2 especially, are any indication, then I think the devs are aware of that. Despite appearing on inferior hardware, they did more than just scale-down the SNES for the visuals. A lot of it is clearly based off of X4. X's teleport animation, ghetto dash shadows, nearly all that is Zero, and even one of the fortress stages (which is rather odd when you consider that near all of the bosses are lifted from SNES).