2d is the best way to go IMO so i'm good with how it is now. i remember playing one of the first 3d harvest moon games and just being overwhelmed with everything. sdv to me is better suited, both artistically and gameplay-wise, to 2d
if you want some 3D you can play farming simulator or at least minecraft with some mods
it's interesting to see how 3D would look like... but i think charm of this game is in his 2D graphics. It's easy to run on all devices (even with old ones). it would be cool if they will add more interesting story or maybe DLC idk.
conclusion is that it would be intresting but what we have right now the best option.
I think it would lose a bit of the essence of Stardew Valley, if they did, I'm afraid it will end up as Harvest Moon Light of Hope, but if they did, I hope they try hard to keep it from looking like poo xD