I am an occasional user of Mathcad, so adding many new features might not be a big draw; one advantage of a limited feature set is that it is easier to pick up old work without getting to involved with relearning all the functions used.
I have not reviewed the Prime feature set, so these items may already be covered, but some annoyances with Mathcad I have are:
Graphics insertion is not the greatest. While some have asked for a sketch tool built in, it will likely be yet another tool to learn to use. Inserting in GIF or SVG format might be better as there are a great many open source drawing tools to produce them. Bitmap is, of course, too much data, jpg is okay, but maybe improve quality. jpg into word documents seem to look better than jpg inserted into mathcad. png format does not work much better for me. svg might be a good choice, but I do not get good screen viewing quality inserting svg type documents into word even though they print okay. There may be a Windows OS issue there.
Moving around the document. The mentioned tab format looks like a bit improvement. While the top down format is the simplest and straight forward, sometimes seems it would be more convenient to have the inputs and results at the top and show the calculations following. Scrolling to the bottom can be tedious in a large calculation. Are hypertext-like links already available within a document?
3. As well, is there currently a way to call another program from within a page. That is, could I do a file output function, follow that with a command and argument line that calls some kind of command line computation, and then follow that with a file input which reads file produced by that command line? This seems like a very basic function that would be easy to implement. I am aware that is is possible to produce .dlls that can be added to the function library, but a command line option opens up the possibility of tcl, perl, etc.
4. (Why stop at 3 because asking for functions is so much easier and fun that actually writing and testing them yourself). Options for giving labels for table displays (other than using the first row and column as text) would be a useful low effort function to improve readability.