This. Although I worry about the longevity of my 3DS, and of my SD card...
That, and just exploring everything again. I've played a few challenge runs more recently, but I haven't used the actual cartridges in years (since I never replaced the batteries), and I always feel more connected to my games when I play them on an actual console. I'm looking forward to having something "new" to play before bed on my 3DS, something I'm familiar with and don't need to think too much about.
I'm also excited about getting more into gen II breeding. I did a little bit at the time (that's when I first learned about egg moves, and it's also when I learned about stat experience and competitive battling), but not much, so I want to take a more competitive approach this time around than I did the first time. It's too bad Crystal's battle tower won't be available, nor will I be able to use my teams in Stadium 2; it would have been fun to have more uses for competitive gen II teams.
I worry that I'll be disappointed, because the games won't feel the same. I won't have my same team, most likely, and I won't have the same ID number, either. And there was something awesome about exploring Johto blindly for the first time, and learning a lot as a trainer along the way, and that won't be there. Regardless, though, I've had a really great time with the RBY releases, and I've put more time into the TCG game than I ever put into my original cartridge, so I'm sure my hours on Gold (and Silver) will be fairly high as well.