Hi,
Watopia should always be available. It’s by far the best course and with all the new roads it has something for almost everybody. London and Richmond is way to limited. Whenever i want to ride a climb, i want to ride a climb.
100%.
Ideally you should be able to seamlessly ride from one world to another.
Failing that Watopia should always be available with an alternative course of the day (be it Richmond, London or something new) available as a second chocie.
We try to add some variety for each of the courses so that they all have some climbs and some flats, etc. Is the problem that it takes more time to get to the climbs on the other courses?
As for the general question here, we’ve been holding back on allowing course selection so that there’s always someone to ride with. One of the big issues we saw with other online cycling programs was that having a wide variety of courses meant you’d only have a few users on each course. By keeping to our current calendar schedule, there’s always someone online - and Zwift is all about being social.
That being said, the plan has been to explore course selection in the future after we hit critical mass and build the UI to support this, so stay tuned. 
I think a great option would be to connect all of the courses, but give us a better view of how to get to each no matter where you are on the course. A map that would appear like a top view of the map would be ideal. (I second every request for a top view map.) I love zwift, but I don’t always remember what the signs covered by arrows mean. I like Watopia the best and tend to skip Richmond, but wouldn’t mind riding part of one to get to the other.
I’ll be looking at signing up again soon for this Winter, last time I was on this a few months back it was teaming with riders, I hope by now that being able to swap between Watopia, Richmond and London is now available? If not it really should be, I like the idea of being able to ride a couple of laps of each or as mentioned above to be able to combine all three for a world circuit, at the very minimum circuit selection should be available now, it’s certainly popular enough.