Will any more consoles be added to the NSO service?

[UnderwaterFunktown]

I would imagine they will carry over the service in some form to their next system after how much time they have spent building it up, very possibly with a name change, and in that case I fully expect more systems will be added at some point, if only a couple.
As for whether there will be added more systems within the lifespan of the Switch, I doubt it though, particularly since the GB and GBA started out with so few games, it'll take them quite a while to really build up those libraries.