I'd rather wanna see a brand new title on the Switch, but if I had to choose one, I would say WarioWare DIY Showcase because it's one of the lesser known titles but still quite good, and deserves to have a larger audience. It could come combined with (a vastly improved and expanded) Snapped, preferably in format that makes it more interesting to newcomers to WarioWare.
They're not the kind of games that I feel would really benefit from getting the remake treatment personally, given the nature of the series. (Plus and this is just a me thing, but I think most of the Warioware games have aged perfectly fine today despite some of the occasional rough edges.)
Plus, Gold has already kind of fulfilled that criteria IMO, okay, the scenarios are new and there are new Mircogames added, but a large part of them are basically remakes of older Mircogames, so...
At this point, I just want the series to make a brand new title with all new Mircogames that takes full advantage of the Switch's capabilities instead of going back and remaking an older title, as it just doesn't seem necessary to me.
However, despite all of that above, there is one Warioware game that I honestly wouldn't mind seeing a remake of, and that is D.I.Y.
Considering the original's online mode is dead and D.I.Y Showcase is also dead (legally anyway.) And current techonology has advanced significantly since 2009/2010, a remake of D.I.Y can really push the idea to it's full potential, but even then...no reason why they can't just make a straight up sequel or what not to D.I.Y that evolves and expands upon the idea compared to the original game.