Altered Emerald is my labour of love - a somewhat-popular mod of the original Emerald game, for GBA.
I've been tinkering with it since June 2013, and just recently I wrapped up with one last patch. It's a nostalgia trip through Pokémon Emerald, with access to all 386 mons (and their gen 4 evos) alongside all the moves and abilities up to gen 6 and 7!
I've also updated the routes, and added about ten more (with a massive island accessible from route 109). Also Nature Power works in place of all HMs, there's an EV checker, a nature swapper, contests, an ability that makes the type chart invert, and more.
I am a massive nerd about this game. It's free, and you can get the .ips file here: Altered Emerald 4.2.
The download contains an .ips file. You can make an Altered Emerald using Lunar IPS, clicking "Apply IPS Patch", and applying the IPS file to a clean Emerald rom. You can play that patched rom with VBA-M.
Remember to obtain this Rom legally, and not through an illegal download like the one at emulatorgames.