Something I didn't like in DOS2 was that the only time you could 'bring' your own character into your friends game was at character creation.
First option; they should allow you to create and save a character , and then when when you join another game at character creation you can quickly use your previously generated character. This saves time etc. This should be in the game at launch.
Second option: more thought required, but allowing you to place your, for e.g. 4th level character into your friends existing game. Other games allow this. For balance have them in the same act. Or only when you start a new act/ chapter of the game. Have the restriction that all characters must be within 1-2 levels. Would need to work out the journal/ quest system.
Many players find it difficulty to play the whole (50+ hour) game with the same set of people due to work, study, family etc commitments. This would give more opportunities, and more fun.
Being able to 'transfer' characters around would also help with DM games as well.