A stadium for DS huh..? Well I guess it would be fun in it's own way. I can't say of course that they could make sth like that, but a game in which you can choose ready made Pokemon and make battle with WiFi or make the battle in Stadiums and even better have mini games would be just cool! However not the old Stadiums since sth new would be really better.
Just think a game with features like choosing between all Pokemon and these Pokemon are already made by system. You can have battle with any tupeof Pokemon tou want or for example battle with babys or fire Pokemon, it would make it really easier than making Pokemon that we would not choose for our team (if you get what I mean). Another nice feautre would be Mini Games, like the ones from Stadium. Really awesome But just dream in my opinion.