One world only would be a dream come true. Time shifting doesn't sound great, but as long as it doesn't have load screens between time shifts it could be alright. For example if you have a Time Visor.
I forgot about this and the time shifting mechanic. We should be focusing on this.
I want only one world so that's good to hear and I loved Metroid Prime 2. I had no problem with the dark and light world. It's my favorite of the Prime Trilogy. Obviously shifting time is different and would be super cool to see how they fully implement it.
I'd like to believe that timeshifting would be incorporated more seamlessly into the gameplay. For example time-bombs that slow down time in a small area or a timebeam that basically works like the ice beam by stopping objects. Having two parallel worlds at all times like that segment in Titanfall 2 probably wouldn't work but I wouldn't want it to basically be a light and dark world with a different coat of paint.