Definitely not "all regions", for a regular Pokemon game. When they are ready for an MMO, then it should include all regions, but the only other context in which an all-regions game would be a good idea is if they decide to reboot the series (as between Gens 2 and 3) and have more than 100 for levels, for instance.
On the other hand, it's certainly time to start considering including multiple regions in each game, as a matter of course. For instance, if they decide to do a Wii U pokemon game set in Orre, they should also include Kanto (done in full 3D) as a post-E4 region (in my hypothetical situation, Orre would have an E4 by the end of the main story). The inclusion of Kanto in Gold/Silver should have been an idea that continued into future games, and the way they handled it in HG/SS was an improvement on the original. The story should continue into the new region, too.
Actually, that's probably the one thing about ORAS that slightly disappointed me - it was a perfect chance to incorporate the Sevii Islands, and they could even have linked it to Soaring In The Sky, to make it work well... and then the Delta Episode stuff could have worked in some Sevii Island content, too.
But while two regions is definitely a good idea. Six would be too much. Including all regions would be bad.







