This might get buried, but I'll put this idea out there.

Background:

I'm someone who has traveled fairly regularly (> 30 countries), with some trips comprised of solo backpacking across Europe or South-East Asia. I also think I'm really good at planning, which is largely about constraint optimizing and solving some version of TSP (traveling salesman).

The idea:

You're in a city. You tell the app all the places you want to check out. The app returns an itinerary, calculating TSP starting from your lodging, taking into account expected travel time, the times the places are open, lunch / dinner, etc.

I would personally find this really useful because it would replace hours that I spend optimizing my travels.