I think I can field your questions haha. At least the first one.
Pokemon tends to alternate a bit between focusing on "additions" to previously-discovered Pokemon and focusing on brand-new Pokemon. The big "new stuff" gens were mostly the odd ones, as you pointed out -- Gens 1, 3, 5, 7. The ones that did more augmenting of the past were mostly Gens 2, 4, and arguably, 6, although I think 6 straddled the line a tiny bit more than Gen 2 or 4 did (although I'm sure many will disagree, and I can understand that too).
Whether Gen 8 will follow the trend of the even numbered Gens remains to be seen. Sadly, though, I wouldn't hold my breath for new Mega Evolutions. It really does seem to be something unique and special to Kalos; but you're right -- with the big influence that Mega Evos have had on VGC and competitive metas in general the past while, to some people it would be nice to have some new ones to add to the roster rather than the creation of yet another mechanic (ex: "Armoured" Pokemon that can attain mega-esque stats) on top of Z-Crystals and Megas for players to think about. Fortunately or unfortunately, depending on how you see it, we're more likely to get a new Pokemon-boosting feature (ex: Armour) than new Z-Crystals or Megas. But hey, anything could happen!
Edit: I know nobody asked, but PLEASE GAMEFREAK, I know you said it wouldn't return because kids these days don't have the patience to grind through it and appreciate it, but please bring back the Battle Frontier ;__;
I do hope we get some change that looks at old pokemon. I always felt Gens 1-3 in particular need the most help just because their stats were designed pre-Physical/Special-split and thus are the most unusual.
Pokemon just having stats that really don't match their typings or movesets at all because of the previous way things worked back then, or being visually very cool, but their stats/typing/ability don't really properly reflect what they're supposed to do.
I'd love it if in one new Gen, the big thing they did was look at old pokemon and just do a mass rework, making up a bunch of new moves, abilities, and applying minor or significant stat changes to certain pokemon to bring them more in line with how their stats would have been designed if they were made post-Physical/Special-split.
For example, Carnivine is a Venus Fly Trap that is weak to bug; it should have some ability that's like storm drain in that it takes no damage and either inflicts damage, gets healed, or gets some sort of boost when hit by bug moves. This ability could also be given to Victreebel as it is also a carnivorous plant (pitcher plant).
Tropius, in addition to either a stat rework/boost or evolution, could have some new berry synergizing moves such as a special attacking move that consumes the holder's berry and activates a different effect depending on the berry. Certain ones would do their normal effect (heal 50%, boost crit ratio, etc), while others like the resistance berries would cause a powerful 120 power 100 accuracy move of whatever type they're made to resist (i.e. Haban Berry causes a 120 Special Dragon move with. This move could either be a signature move for Tropius, or just be learnable for all pokemon that can get the Harvest ability, effectively buffing Exeggutor, Trevenant, and Tropius at the same time.
