Last year, I've played through Red, Yellow, Gold, Alpha Sapphire, and X (from start to Elite Four). Each time with the same two restrictions/goals:
1) Catching 100 pokémon before challenging the Elite Four.
2) No legendaries allowed in my team to beat the Elite Four.
It was especially challenging in Red (Yellow was easier because you had the 3 starters). I've played through the gen 1 games dozens of times at this point, and I think that's the first time I managed to beat the Elite 4 without using any of the legendary birds.
This year, I wanted to complete the Platinum national dex. So before that, I've replayed other games: FireRed, Emerald, and HeartGold. All with the same restrictions as the previous ones, in order to catch as many pokémon as possible and build strong teams.
Started replaying Platinum in the last few days.
Will probably also replay Crystal, Diamond/Pearl, and White at some point.
Haven't played the gen 7 games, so I can only contrast with gen 6 (Alpha Sapphire & X). I'd say that the main differences are:
- The graphics. Of course much better in gen 6, I love the 3D pokémon, the battles, and the level of detail in places such as Lumiose City.
- The difficulty. Gen 1 to 4 games are way more challenging (can't speak for gen 5, though, as I've only played through White once and don't remember much of it).
For those of you who do decide to start over, do you trade your old team over to other games?
Yes. I've done it dozens of times with FireRed & Emerald by transferring my teams to the gen 4 games. In Pearl I have a PC box with quite a few Charizards xD