How to increase the population in a few easy steps
A few ideas from Wizard Central
1. Advertisements and actual gameplay trailers not CGI. Advertise in other countries, not just the US European Countries would be nice.
2. Upgrade graphics in earlier worlds. This game is aged to kids and teenager who actually care about graphics. Just polish them and that should be good enough.
3. More story in general
4. The most important thing is to listen to the community when several things have been said to be fixed, changed, or removed repeatedly, do it because if the fans hate the game, the less people who want to play your game will overall decrease.