I deliberately express startup ideas as x for y whenever possible because that’s the most efficient way to describe a startup in a few words. Airbnb, for example, was in its day “eBay for space.”
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).