GameCube, but the 3 Game Boys would happen first.

The GameCube is sorely needed because it has never been on Virtual Console or something similar (even the Wii had digital games on the Wii U). Also, GameCube games are really high in price online. Granted, that's not a uniquely GameCube issue.

GCN, Wii, and Dreamcast are far and away the most unlikely of the options you gave. I would not put it past Nintendo to have a third tier (an expansion of the expansion pass) for those platforms.

PS5 and Wonderswan for sure.

Gamecube. It also just makes the most sense. Gameboy games can already be played on the go (legally) while Gamecube would be a new and shiny feature that everyone wants. Gamecube would sell way more subscriptions than gameboy. I think Gamecube will come next and it's also what I would prefer to see. Currently we are getting Gameboy games in remastered form and I see this trend continuing. 

@Wman1996 I agree that GCN, Wii & Dreamcast would be a third tier. Would also throw in DS as well. I actually think it will be a third tier exclusive to the next system. Maybe 3DS would be included as well depending on how long the next system's life is.

I strongly want the Gameboy Color Advance series more, So many awesome games like Wario Land series and Donkey Kong Land series ETC Lufia and other RPG that AAA rd parties will port since most of those games are not Flagship titles like Final Fantasy 6, or Secret of Mana etc.

Dreamcast

I need Space Channel 5 in my life

TBH,Dreamcast library is not big and most 3rd parties wont port their mega games since they'll likely do a remaster and cash in on hype alone games like Skies of Arcadia need a physical and remaster I prefer physical than a service which will disapear like that awful cloud version of games once cloud is gone so is the service of games look at google stadia etc ., It'll be easier to convince 3rd parties to port gameboy series since those wont bank on hype 

I want to play SPACE CHANNEL 5 in my oled switch ;(

I um, blushes I like that. Maybe they'll make a dreamcast service? but don't expect capcom to port powerstone the hype for that game is above darkstalkers which failed in modern consoles. I think I want Powerstone Physical steelbook. And Skies of Arcadia makes me orgasm of a port with physical edition Sega knows when the right time it is to make Skies of Arcadia for Switch hype of that of Chrono Trigger.

GB and GBA this gen, maybe Dreamcast and maybe GC next gen. I don't see any other systems being added.