Just as Red and Blue had Yellow, Gold and Silver had Crystal, Ruby and Saphire have Emerald. Keeping the tradition is infamous in the Pokemon series, and for all the right reasons.
Emerald takes in same place and time as Ruby and Saphire (R/S). However, the story has been revamped to approve a new game. R/S owners will remember that you could only get Groudon and Kyogre in one or the other game. However, in Emerald, you can get both, including the other legendaries and more to boot.
If you remember, R/S featured 2v2 battles, but few were present. In Emerald, however, if two seperate trainers see you, they will team up on you. A great improvement indeed.
What's great is that Emerald features two very replayable additions. Battle Frontier is accessable after the Elite 4 and has 7 battle grounds, all with different ways to rise the ranks. Very unique. The other is the pokedex upgrade that reaches up to 384! Incredible. You can have EVERY pokemon ever in your control. :)
Some R/S will think not to get this cause it's a remake of them, but they should get Emerald even more.