I come from the possibly unique point of view that above all I think that you've made a stellar game that I really adore, and I want you to get paid handsomely for that, above all else.
To that end, I think it would be criminal to release a free London-only version of the game. You'd get huge downloads, but very low conversion, and you'd not make nearly the money you could. I'm reminded of OneManLeft's Outwitters, which had 4 diverse teams, but one of them was free, and almost everyone was satisfied with that. I've been addicted to this game for a year, playing London-only, so I know that plenty of folks would stop there.
I also think that it should be possible to buy into 100% access to the game and not have to confront ads if bought.
So where's the middle ground? I'm not at all bothered by Crossy Road's implementation. I've not bought a thing, but I watch the ads (or run them and pay attention to something else for 30 seconds) to speed up my unlocking. Radical's "watch an ad to continue after your first fail" also seems to work for me.
The problem here is that Mini Metro isn't an unlock-fest like Crossy Road, and watching an ad to continue is counter to the spirit of the game. So you'd likely have to come up with something else. But I say the free version should have only one map AND ads. That way you get your money either way. And the upgrade is doubly attractive, offering more maps AND removing ads. Win-win.
I also think anyone that's been playing this game and is passionate enough to comment on this forum is lying or low-balling if they say they'd only pay $4. I know I'd pay more.