Replicating the basic features of an oscilloscope would be tremendous. My use case for oscilloscopes is mostly leaving them running while I swap parts in and out on a breadboard. This sort of interactive use is infeasible without live display. For this, I don’t need any measurements, only the visual display.
Since you asked about aspects where Saleae could go beyond traditional scopes, I’d first like to say that 8 channel scopes cost as much as a car and 16 channel might not even exist (if they do, they’d cost as much as a house).
And then there is memory depth. This is where Saleae really pulls ahead of scopes. Two of the items Pat mentioned, 3c (decay) and 3e (step through history), could be combined. That is, you have a slider that chooses the n^th capture to be brightest, and the captures before and after that are progressively dimmer. I think this would be visually striking and could end up being useful. For instance, you could easily see whether the captures before and after a glitch were abnormal, and you could see how a signal has drifted over time. These captures should be timestamped.
The XY display on my entry level Rigol is terrible. I think there are two reasons: low memory depth and a gap between captures. I could see making a very nice XY plot on the computer. In fact, it could even be an XYZ plot you could rotate in 3D. I don’t think anybody else does this. I can’t name a use case, but I’m sure it’d get used. XY plots are for things like voltage vs. current, or voltage into a filter vs. voltage out. A third channel here is useful for the same reasons it’s useful elsewhere. Or, if not 3D, it could be plotting XY, YZ, and XZ. Of course, this is gravy. The biggest benefit will be from the traditional basic scope features.