For starters it'll have to have two seats - the lighter weight of a single-seat machine might appeal from an engineering point of view, but the market for single-seat aircraft is about as big as their cockpits. I don't think that high speed will be too important, but lack of speed will be a bit of a dampener. 70kt is marginal, 90kt better, 100kt ideal. Range/endurance - it has to be at least a couple of hours surely? Anything less is going to be a real pain, and three-and-a-half to four hours would be ideal. Training aeroplanes will either need a quick charge or a quick change of batteries.
So, two people, for two hours, at 90kt and that, I think, is the bottom end of the acceptable range.
I've heard people predicting that in ten years we'll have four-seaters flying for four hours at a time - and if they manage that at any decent speed (120kt at least) we'll be in for some interesting times.