There are rumors circulating about Game Boy games coming to Nintendo Switch Online, and there are some titles that deserve to be on the service on day one. The Game Boy and Game Boy Color are the third best-selling video game console of all time, which means its library is more than deserving of appearing on the Nintendo Switch Online service.
The people who subscribe to Nintendo Switch Online get access to a selection of NES and SNES games. When the service first launched, fans assumed that other consoles would be added over time, though it's yet to occur. Nintendo does occasionally release Nintendo Switch Online exclusives, like Pac-Man 99, but no new retro systems have been added to the service.
There are multiple sources claiming that Game Boy and Game Boy Color games are coming to Nintendo Switch Online. It's likely that Nintendo would kick off the service by adding some high-profile titles, and there are some that are likely to be there if Game Boy games launch on the Nintendo Switch.
Super Mario Land 1 & 2
Super Mario Land and Super Mario Land 2: 6 Golden Coins would be all but guaranteed to appear on Nintendo Switch Online. They are two of the best platformers on the system and they still hold up to this day. If Game Boy games are coming to Nintendo Switch Online, then Mario's two main adventures on the system should be there on day one.
Tetris
Tetris is the game that made the Game Boy, while the Game Boy also made Tetris in many ways. The only pack-in games that come close to being system sellers on the same level as Tetris are Wii Sports and the Super Mario Bros./Duck Hunt combo cartridge. Tetris might not have all of the features of the modern incarnations, but its gameplay and soundtrack are timeless - it wouldn't be a Game Boy collection without it. The only reason Tetris wouldn't be a priority is that Nintendo Switch Online subscribers already get access to the excellent Tetris 99.
The Legend Of Zelda: Link's Awakening
The Legend of Zelda: Link's Awakening is one of the best titles on the Game Boy. It was Link's first bite-sized adventure, yet still packed an epic quest into a Game Boy cartridge. The dream-exploration story is one of the best in the series, with some phenomenal dungeons and lots of secrets to uncover. The main reason why Link's Awakening wouldn't appear on Nintendo Switch Online is that Nintendo is still selling a full 3D remake of the game for the system, and having a free version available could diminish its appeal.
Pokémon Red & Blue
Pokémon Red & Blue are the games that kicked off the Pokémon franchise, which is the most profitable media entity on the planet. The world is going through a Pokémania nostalgia boom, and the ability to play Pokémon Red & Blue on Nintendo Switch would be a major draw for Nintendo Switch Online. The main reason why these games wouldn't appear is they're owned by The Pokémon Company, of which Nintendo is only a part-owner. The Pokémon Company knows it could make a lot of money selling these games on the eShop with online functionality like they did on the Nintendo 3DS, so it doesn't make much financial sense to put them on Nintendo Switch Online. It could also be argued that The Pokémon Company is selling the Let's Go games, which are a remake of Pokémon Yellow.
Donkey Kong (1994)
Donkey Kong (referred to as Donkey Kong '94 by fans) took the concept of the original Donkey Kong arcade game and expanded it, adding lots of platforming puzzles that are spread across nearly 100 levels, and giving it a Super Mario Bros. 3-style world map. It's one of the most highly-regarded titles on the Game Boy and it will likely be one of the first games added to Nintendo Switch Online.
Metroid 2: Return Of Samus
Samus' second adventure was a major step up from the first, sending her on a mission through claustrophobic environments, and pitting her against a group of Metroids that keep growing in strength. Metroid 2: Return of Samus might have the best chance of appearing on the Nintendo Switch Online service (outside of the Mario Land games), as Metroid Dread is coming out in October, and Nintendo will want to promote the series as much as possible in the run-up to its release.