The routes. Hoenn is my favourite generation for how it was laid out and mapped. GameFreak have never made a game with such a perfection region since (even despite the water which people complain about nonsensically hehe).
Route 119 is one of my favourite routes and areas in any game. To me, it was the most well crafted route in GameFreak's history. I'm excited to meet May/Brendan just before that bridge, then cross it right next to the waterfall.
I'm also really looking forward to seeing how they make
Ever Grande City look. It's such an aesthetically pleasing area in Hoenn, and I always thought it really set the theme so well after all the chaos that went down with Kyogre and/or Groudon. Seeing that area in 3D, I can't wait. Not only for the visuals, but to remember that feeling of success or achievement before you go through Victory Road, onto the league.
There's a lot of areas of interest I want to see too, like Meteor Falls. Which is such an amazing looking place in R/S/E, I'm really excited to see what they do with it in 3D. If X&Y is any inclination as to how well a cave can look; it's going to be incredible. Same thing goes for the underwater routes. I am hoping that we find a Manaphy in Hoenn, so if that's the case, I'm really excited to see how they would stage that event if it was under the sea. So I hope they expand the underwater routes a bit, and litter them with some more points of interest rather than just shards (which are pretty boring).
Hoenn was a region that brought so much diversity in its towns and cities. Look at Fortree, Lavaridge, Sootopolis, Lilycove, and Pacifidlog. In those areas alone, there is so much diversity. I can't wait to see each of those in 3D. Especially
looking at Fortree in the Anime.