I'd start with New Super Mario Bros All Stars as a way of filling out Switch's library and leaving that series behind (rather than just porting U and Luigi U), then doing what a lot of people are suggesting here. The basic level design and controls are fine, but a bolder take on the aesthetics, music and selection/variety of content is needed; 'New Super Mario World' would be good, if they go ahead with those changes.
You know, I didn't even think of an NSMB All Stars collection, but THAT might get me to pay $60 for it, if they still did some bonus content. And All Stars on the SNES heralded the end of the Mario Bros. line and made way for Super Mario World, so doing so now would be a good way to simultaneously offer enough content to make people feel like the port was justified, while also sending a message that NSMB is wrapping up so that people won't think the port means even more NSMB is coming, and would get excited about the possibilities. I imagine they probably would call the new one "New Super Mario World", because I doubt they could make a better name, knowing them, and it would get the idea of the direction they're going across, if they decided to draw more inspiration from SMW for the new one, which I think they should.