1) Proper scripting support. Python or even VB would be great. This would allow me to program custom functions inside of Mathcad using techniques I already know. I don't like the Mathcad programming techniques provided in M13, and am even tempted to revert back to MATLAB to process data.
2) Means of interfacing with electronic test equipment. Again, Python allows me to use things like PYVISA to talk to hardware outside my computer. It would be fantastic to be able to use this to write Mathcad based test script worksheets - that is it would set up the equipment, read the data from a scope or spec analyzer into Mathcad and then process it immediately instead of post processing.
3) I hate to admit it, but I like the ease of manipulating block diagrams like in Simulink or Labview, and wonder if something like this can be done in Mathcad. For example the ability to put a Mathcad worksheet into a collapsible calculation block with input and output connections. Then importing and even stringing several of them together in another higher level worksheet.