Emerald is sublime! It's one of my top 3 fav Pokemon games.
Since Emerald and Leaf Green are both part of the third generation of games I recommend Heart Gold or Soul Silver as your next conquest. Heart Gold and Soul Silver are fourth generation games so this would place you in the right follow up generation to the games you last played.
There will be new Pokemon moves to learn of course and some moves that were previously only physical or special have been reclassified. Yet while this is a generation that introduced important new mechanics i think Heart Gold and Soul Silver would still be the easiest 4th generation game to start with because you don't need to learn a lot of new Pokemon right away. Since this game is a remake of the Johto games like Gold, Silver and Crystal the main story stays with pokemon species and typings that you would already be familiar with from previous games. This makes the game very approachable and nostalgic.
Its a very, very well-liked game. The graphics are a beautiful mixture of what you have seen previously in gen 3 but with a little 3d incorporated it. The sprites are adorable and it has the unique feature that you can make any Pokemon you place at the head of your trainer party follow you around, you can talk to them at all times, they bark, play, cuddle you, fall of sleep, collect objects to share with you. it makes these games really unique and fun.
The story is more developed than the original GSC games and there are many new features, a safari zone, a pokeathalon where your Pokemon can get in shape and participate in fun mini games, the battle frontier from emerald is back, the post game is elaborate and allows you to catch almost all of the legendaries from earlier generations and re-visit Kanto in a really involved way that will be like playing Fire Red/ Leaf Green again.
The only thing is that this the game can be a little tricky to get nowadays so I would recommend either buying it used or playing on a free emulator. However, if you manage to get a copy I think you would be really happy.