Most promising things on my mind these days:

1. Any app that lets users buy stuff they cannot buy on Big Tech platforms.
- DVM clients, especially combined with GPUtopia etc
- Marketplaces for farmers with reputations, recommendations

Reason: Real money coming in at scale

2. Apps that integrate P2P communication with relays: Overlap of Nostr & Holepunch. 
(Instant Giga-File sharing, Superior quality streaming, podcasting apps, ...) 

Reason: better quality & faster

3. Any app that lets users make decent money and combines it with easy ways of spending it. 
- Interactive Subscription based newsletters/blogs/courses maybe
- AirBnB for bitcoiners or some service like that, where you'd want to select for like minded customers and could actually use the existing nostr social graphs + significant sums of money circulate

None of these really exist yet though. Of the existing ones I'd say:
- Amethyst (if it does P2P)
- Highlighter (if it integrates patrons)
- Zap.stream (if it does P2P)
- Spring (if it specialises on onboarding)