The GBA is a classic 2d system underneath it all. This means the widescreen stuff seen in the likes of PS1 emulators on up (or indeed https://www.wsgf.org/
on the PC side of things) where you just render more of the scene is not an option - 2d is sprites and arrangements thereof, not to mention the logic will often go based upon that (how many things will t-pose until you get to their original render distance, or possibly visual cone, in 3d, or start walking as soon as you get on screen/step on an imaginary switch).
This also says nothing of the problems of https://docs.google.com/document/d/1iNSQIyNpVGHeak6isbP6AHdHD50gs8MNXF1GCf08efg/pub?embedded=true
Emulators doing it for the GBA will be doing one of two things.
1) Pausing the emulator every frame, pushing the direction buttons, grabbing a screenshot and stitching it all together. Works fine for some things but random battles triggering, various animations/overlays (though they might literally be an overlay), being stopped by a wall/npc/cutscene and possibly random battles themselves if take place somewhere else as it were.
2) There is probably some kind of camera function (see the link above for some takes on the matter) which you can find more easily but will possibly mean delving into a game a bit. If it helps to imagine then basically a tweak on walk through walls/disable collision physics. Force this backwards and forwards (and side to side, might even have a nice means to get diagonal shots to get the corners and spare yourself a few extra rendering steps each time), possibly disable random battles if you are going to go game specific as well.
Each of those has a few extra tricks you can do (starts with things like setting the background colour to the background colour of your image rather than white, not unlike those videos where people scale or account for sufferers of vertical video syndrome by playing the same thing in the background but maybe slightly blurred, to rendering a sprite that might normally be chopped off by the edge of the screen, and goes rather more in depth with some kind of logic trigger detection akin to http://fceux.com/web/help/fceux.html?CodeDataLogger.html
but to detect things and differences) but none of them are magic bullet territory or ever really going to come close to the stuff available for 3d based devices.
It is not impossible to do a hack that widens the scope of a game, and some games might indeed have an extra tile or background in use ( http://problemkaputt.de/gbatek.htm#lcdvrambgscreendataformatbgmap
says 512x512 in one of the modes for a reason) or possibly even prerendered to make life a tiny bit easier, but it is going to be a nightmare hack probably not so very far off porting the game out, and that is before you find the game devs knew the resources they had to play with and pushed it to the limit. Do some emulator specific hack/combine it with an emulator specially recoded and you could make that a bit easier, however nobody likes those approaches really (see all the fun with people not being able to do things on portable devices now they got big)