I mean, it's not even a stretch to say that most of the biggest GB games have already received the fan DX treatment as it is: All the Super Mario and Wario Land games, all 3 Donkey Kong Land games, Pokemon Red and Blue, the original Kirby, and Metroid II.
What other non-color Game Boy games are the popular heavy hitters that haven't received the treatment? obviously games like Link's Awakening received the treatment officially, so that's off the list too. In terms of the popular games that still haven't seen it, all I'm thinking of are Donkey Kong '94 and Kirby 2. That's pretty much it. Maybe Kid Icarus 2? Not exactly a heavy hitter, but a first party series with its share of fans I suppose. I guess there are third party games like the Mega Man entries, but even the well-love OG Game Boy third party titles are few and far between.
Also, incidentally, while it isn't a color DX hack, there was the first complete rom hack of DK '94 that just came out the other day which includes over 25 new levels made by a community of fans of the original game. Gonna have to give it a go shortly here:
https://www.romhacking.net/hacks/7213/