The only problem with going back to Unova is that every Unova pokemon would have to return (apart from maybe some legendaries) or it would be strange. How do you explain half (or more) of Unova's pokemon being replaced by Kanto, Johto, Hoenn and Sinnoh pokemon?
It's not the same as Pokemon Gold/Silver where there was only the pokemon from two regions to consider.
My ideal situation would be that they would sell additional regions as dlc on the eShop for about €10-15. I would only be willing to pay if it included a full experience though. That means new main storyline and post game, evil team, rival, gyms, elite four, additional features and aspects that make each region what it is (if I buy a Sinnoh expansion pack I want to be able to make poffins, if I buy the Hoenn expansion I want to be able to enter all the contests or set up a secret base in a tree, if I buy the Johto expansion I want to be able to enter my pokemon in the Pokeathlon etc)
I wouldn't expect to get a full region for free. I don't think it would be worth their time.
My expectation is that they won't attempt something like this for the 6th gen, but it would be amazing if they did go down an expansion like route.