Clearly someone isn't paying much attention to the service if they think EarthBound is still absent
The idea that Nintendo aren't concerned with games outside of their own first-party titles folds in on itself when you consider the tiny, miniscule little factor that they are licencing the Mega Drive library from Sega for NSO. That also includes some third-party games from publishers like... Konami and Capcom! You could just spend 30 bucks and buy many of those very same games on the eShop via the Sega Mega Drive Classics compilation; doesn't seem particularly sensible on Nintendo's part to spend millions ensuring those games remain part of their service, and yet here we are. Those games have value, as do the major absent third-party titles for Nintendo's platforms.
Personally speaking, i'd play the hell out of the NES Mega Man games on NSO, yeah. I already own the Legacy Collection on Switch but NERD's emulation is just superior, and as a huge fan of those games i'd love to play them in the best way possible. They're a core part of the system's library to me (and clearly many others since I am far from the first to share this sentiment), same for Castlevania. NERD clearly recognise the value in those titles too if that leaked build of the NES app is anything to go by; just a shame that their enthusiasm for emulating the best the system has to offer is held back by the realities of licencing and dealing with external parties.
The fact is that NSO is the one and only alternative to Virtual Console Nintendo are offering on their current platform. Wii Virtual Console had well over 600 games; 300 on 3DS and 400 on Wii U; if we're losing that service it's only fair to expect the alternative to eventually offer something that can be seen as equivalent. The four available platforms currently offer a total of 172 games (counting the four announced N64 games that have not released); a lot more have been internally tested but only time will tell how many are released in the end.
NSO's library has huge holes, Game Boy and Game Boy Advance are some of those holes and they will be filled, but Nintendo can't make the service as appealing as possible with just their own titles. I think the above list of N64 titles from CyberWolf emphasises that; there really aren't ton of first-party N64 games left to pull from.
The majority of titles they had planned and tested for N64 VC in the past are not owned by them, and they'll need to start digging into that library eventually to keep adding value to the service. More of Rare's titles, Goemon and Bomberman from Konami, Mischief Makers for the cult classic hit; those are the types of titles that'd enrich the library and make it more appealing to different groups of fans.
They should also improve the quality of their online service too! These two goals should not need to preclude each other.
To briefly weigh in on the current discussion; i'm currently betting on GB(C) for base sub and GBA for expansion pack. If anything I think both being on expansion pack is more likely than both being on base sub. We'll just have to see when Nintendo feel like announcing them.
This is the year of Doshin's return; I believe!
More seriously, i'm not totally ruling out the posibility of Animal Crossing. The emulator has finished, functioning support for real-time clock emulation which is ONLY used by that one game. Doesn't guarantee it, they've also got mostly functioning Controller Pak emulation disabled for no good reason, but it is very feasible.