Here's a few X for Y things which are likely to succeed:

X for the masses.

X for people who don't like computeres.

X for people who don't speak much English.

X in the cloud (way overplayed).

X for free (but good luck getting a low enough cost of customer acquisition!).

X for people who are locked into Y.

X for large businesses (requires connections).

X for businesses which aren't great at tech (X better be something they really really want).