I think 128GB.
Their digital ratio has skyrocketed over the past 4 years. This is a very good thing for Nintendo and something they will want to encourage. If they think they'll make more money from having enough storage to encourage more digital sales, they'll do it. I predict 128 GB as the sweet spot.
Won't be near enough, though. I have a 256GB card full and another 128GB card full. By the time Switch Pro rolls around I'll need half a TB at least.