Last year when Nintendo released their Corperate Mangement Policy Briefing for the fiscal year, they confirmed that the Switch Online service will carry over to their next gen console. So it'll be awhile before this goes away.
I feel like we should have gotten at least GameBoy games on NSO, and my money is it will be the next expansion of the service. I wouldn't expect a mini GameBoy even though it could be pretty cool. I think a Nintendo 64 Mini would be more likely to release eventually, hopefully with a better joy...