With regards to planets, many barren, lifeless planets are going to come about after our 'big bang', ready to bring to life and populate with our hellish fiends and/or loyal minions and/or dutiful worshippers. I think it would be for the best that we stick to one planet until our creations reach space-faring (if they reach space-faring) technology levels.

As for the actual creation of a new planet from scratch, it is entirely possible. Though it may well be a rather big waste of Might considering there are/will be a nigh on infinite number of planets ready made.