Here's my thoughts on the revealed stuff:
The Mega Sceptile design looks pretty nice, although I don't think Lightning Rod complements it well as far as abilities go. It its Grass/Dragon-type though which is a combination we've never had yet, although that 4x Ice weakness is gonna suck, alongside other weaknesses of Poison, Flying, Bug, Dragon and Fairy. Yeah it gets pretty weakness-heavy so it's essentially gonna be hard mode for anyone who picks Treecko.
Mega Swampert on the other hand looks really ugly with those huge fists, almost made me think it became a Fighting-type at first but apparently it remains Water/Ground. Swift Swim was an interesting ability choice though, but I guess that can help overcome its low Speed in the rain. I was going to try and stay true to my pick of Mudkip in the original but now I'm not so sure, if it is too ugly I might actually end up picking Torchic this time around (I really do like Mega Blaziken's design plus Mega Sceptile looks kinda shafted to me), given that I go for the Fire starter if I don't like the Water starter's design. I do like Mudkip's line but Mega Swampert will take longer to grow on me than I first expected.
I love the new uniforms for the Team Magma and Team Aqua grunts, although I have some complaints regarding the new Team Aqua designs. First off, they are all dark-skinned now (I guess Game Freak wants to make more ethnic diversity with the characters but now they feel so weird compared to the original) and I don't like that new male Aqua Admin design. Archie does look really cool though.
A Mega for Diancie was also unexpected, but I like the chandelier-like design there. Still, it looks really good and no disappointment design-wise there. It's always what I always imagined for it should it have gotten a Mega.
And now the protagonist redesigns which I have mixed reactions. I really don't like Brendan's new clothing, it looks a lot like a wetsuit in my opinion, similar to the one Nate (B2/W2 male protagonist) wore under his clothes. May on the other hand actually looks really cute with her new design, that bow in her hair is a nice little change to the usual hats that seem to be common with protagonists. I know her original design didn't necessarily have a hat, but I love how her clothing is now more in tune with Hoenn being the tropical region. The shorts do remind me of Hilda (B/W female protagonist) in some way though.