I do not know what is in Mathcad Prime so I am at a disadvantage. So my suggestions below are based on pushing Mathcad as it is now. Many suggestions have been forward over the years by many persons on how to advance and make more powerful the Mathcad programme and it would not take long for PTC to skim the old collaboratory to list them all.
1. I work with large data files and I find that Mathcad falls over if am not careful on how I structure the programmes. As time went by, the datafiles kept getting bigger and I know I will hit a complete limit very soon. It has something to do with the internal workings of Mathcad with regard to memory etc. Although Mathcad 15 is much better than Mathcad 14, both are slower and fall over sooner than Mathcad 11 which I still use for my primary work. So my suggestion is that Prime needs to fully work in 64-bit mode within Windows 7 in order to maximise both speed and capacity.
2. Mathcad is poor in producing professional looking documents to present to clients. This covers proper text-formatting under word-processing and also proper presentation of tables. Productivity will be much improved if I can bypass having to switch between Word & Excel & Mathcad. I continue to need Excel but I do not see a need to continue to rely on Word. Alvaro mentioned this as well in the past. So, for me, Prime needs to give high priority to include proper word-processing facilities and also to include better page layout options (in particular, the header / footer option in the existing Mathcad is very limited and also a need to allow for portrait / landscape modes in the same document).
3. I can live with the existing graphs as they cover the basics. But I regard it as very important that the parameters controlling the graph can be programmed by the user.
The basic idea behind Mathcad is really good and like using it. I have gotten used to Mathcad's quirks over the years. But it needs to be made more powerful so the user can do a lot more and it must be brought up to date take full advantage of 64-bit operating systems. I do not want to use what are glorified line-editors such as Maple or Mathematica but I will be under serious pressure to consider them in about a year's time to get past the restraints noted in 1. above (unless I can again find another way to re-structure my programmes to prolong the use of Mathcad).