A Game Boy Classic would certainly be tricky, since making it an actual portable would mean the cost to produce it is significantly higher than the NES and SNES ones. The most expensive component would be if it had a rechargeable battery. I s'pose they could just make it run on AAs, but I'm not so sure people are nostalgic for the battery-munching quality of older handhelds, hee hee.
I also feel like it would be a tougher sell at the $80 price of the SNES Classic, so being able to make it $60 would be ideal. Though I may be underestimating the nostalgia for the Game Boy and people's willingness to pay for these games. With the higher production cost though, it could reduce the number of games in it. I doubt it would have more than 30, but it may be more like 25.
I would expect to see the game list populated with mostly Nintendo titles, maybe all of them if they can't get third-parties on board. If there are a few third-parties titles, my guess is they will be the ones which sold the best on the 3DS VC.
Game Boy Virtual Console games cost less than NES and SNES games.
If you're including GBC though, those games are priced $4.99 or $5.99, the same or a dollar more than NES. And the Pokémon games are priced $9.99 each so I have my doubts they'd be in a Classic edition.