Aside from Rivals, professors and the like, I'd love to see the return of Colress from Black and White 2. Everything about that guy was amazing, his team, his motives and his battle theme (mainly his battle theme). But i was thinking, his main motive was to unleash the maximum potential in his, and possibly all pokemon. We've already discovered mega evolution in this 'second timeline' and have recently been introduced to 'synergy'. As you may or may not know, synergy is a direct bond between person and pokemon and is different to mega evolution in that there is no stone and mega item needed. Now this got me thinking. The use of mega evolution may be down to Colress, as he always wanted to bring out the potential of his pokemon, and so invented these mega items in order to harness the power of the mega stones as he was not able to use the power of synergy. The fact that he is a scientist and this is all high tech development could lead us to believe colress is behind mega evolution.
On the other hand, it could be colress that has discovered synergy as he eventually managed to bring out the potential of his pokemon after many years of hard training. On his travels to achieve this, he may have found no use for the mega stones and scattered them across regions as he made his way across them. With the new advent of synergy, i hope we will see a return of colress in sun and moon, either to show us the true power of synergy, or reveal the true origins of mega evolution and how it came to be (I.E, his creation and how he achieved it)
This theory may be a bit out there, but I hope it is perfectly accepted as I would love a Colress return more than anything