I don't like the idea of new players(low level) joining worlds with far progressed players(high level). An idea world be to condense the worlds automatically based on progression after a period of time so players are on the same level of play. Question would be "how do I play with my friends?". This could be answered by after world switching numerous times, you would know all players and if you stayed to the same progression as your friends you would end up in same world as them.