I think this will not be quite the answer you are looking for, but:
0) I've said this before, many times, but I'll say it again anyway. All the features that are in Mathcad 15 but not in Prime 1.0. Yes, I know that's not "one feature", but that is irrelevant. I, and a large portion of the existing Mathcad users, will not switch from Mathcad 15 to Mathcad Prime until you have all those features implemented because some or most or all of our current worksheets will not work. There are a few features that I personally use very little or not at all, but they will be used by others. For those of us that will not use Prime until all these features are implemented, it makes no difference which order you implement them in! It also makes no difference whether or not you add some nice drawing tools, for example, if we are not using Prime in the first place!
I don't really consider that a new feature though. It's just the implemtation of existing features. So I labelled that (0), not (1).
Once you have got that done, and we are thus talking about new features that are not in Mathcad 15 or Prime 1.0....
1) Multidimensional arrays. Why do we have to ask for these over and over and over again, for years? I would have thought that it would have been trivial to build this capability into Prime from the start. It's only an extension of the current indexing scheme. A lot of mathematical problems, and a lot of data sets, are more than two dimensional, and handling these problems in mathcad is currently very painful because we have to use nested arrays. Why can't I just index a multidimensional array as M[i,j,k, just as I would index a matrix as M[i,j or a vector as V[i ? Even the most basic programming languages (including BASIC) allow arrays with more than two dimensions!
a) Much better graphing capabilities. But they have got to be much better, or this would not be my second choice (see (b)). Visualization of data and math is fundamentally important, and the quality and flexibility of the graphing should reflect this. Currently, in Mathcad 15 or Mathcad Prime, it is not even close to what we really need. If you have any doubt about how important this is look at the success of OriginLab's graphing package!
b) Some sketching tools. These should preferably come with associated features such as grouping (including with other region types, such as graphs), on top / behind, align to each other or to the grid, etc. I'm thinking of something similar to what you get in MS Office. It would be nice to be able to drive these sketches using math too (like we used to be able to do with Smartsketch), but perhaps that asking too much. A lot of problems are very hard to describe or document adequately without sketches or drawings. It would also be very useful for annotating graphs.
3) Compile our Mathcad functions to a library. They should then run faster, and be automatically available within any worksheet. It should be possible to have more than one library, and it should be possible to embed the required functions in a worksheet when I save it (so that it's portable).
We have been asking for 2a, 2b, and 3 for a long time too.