There was a pretty sizable Nintendo Switch Online leak over the weekend, which seems to suggest that Game Boy, Game Boy Color and Game Boy Advance games may be hitting the service soon.
The leak includes Nintendo's own proprietary emulators for these legacy handhelds and a list of games tested on each. Chief among them are Mega Man Battle Network 2, Mega Man Battle Network 5: Team Protoman and Mega Man Zero 3. Although the latter is readily available to play in Mega Man Zero/ZX Legacy Collection, there's currently no legitimate means of playing Battle Network titles on Switch (or any current-gen device).
According to MondoMega, "tested" is the big keyword here. There's no solid indicator that these games will actually be distributed to NSO subscribers. However, Nintendo did get these games up and running on Switch at some point. Whether or not they intend on rolling them out remains to be seen. In the meantime, you can find a more detailed list of GB, GBC and GBA games Nintendo tested here.
On the flip-side, a batch of yet-to-be released "SP" NES Mega Man games were also discovered in a leaked, updated version of the NSO NES app. These "special" titles include Mega Man 1-Mega Man 6. All six SP versions will start players at the Boss Rush.
You can find more details on the NSO NES leak here. Hopefully we'll get to play everything here sooner rather than later!