HAAA... As a man who does this for a living, I wouldn't trust a computer to notate music to save my life. I bet you this is a whopping failure.
Such software already exist, just use a MIDI keyboard and quite a lot of different music notation software packages are able to transcribe what you have played into a semi-useable score (you need to lower the sensitivity by a LOT, and transcribing any complicated rhythm is only going to screw up the system, it seems that computers are just not very good at rhythm). I personally dislike using these software, as the time you need to fix all these rhythmic errors that the software does (and your own playing), might as well be used to write down all these notes on a good piece of manuscript book and/or thinking of newer ideas. I just use my phone to record all my ideas in audio format, for e.g. humming a tune, or improvising on the piano. To be honest, I find that less than 10% of the ideas are really good and it would be a waste of time to put all of them into score anyway. I keep a digital folder on Dropbox to store all my musical ideas. Then I listen to them sometimes, add new things to it, string different ideas together, imagine the structure of the piece and how it is going to sound like, what effect do I want to give to the listener. When it all clicks together, then I write them down, using plain manuscript paper. By that time I would probably have quite a good idea of how to piece would sound like already.
Hey Ted - Would be interesting to see how accurate that improvisation I notated for you a while ago turns out using the software... would be interesting to see the comparison.
What you say about it is true for the novice. If you learn the software it will work for you. If you want it to notate a complicated rhythm then you must play it correctly.
Suppose I were to play a waltz with a Viennese lilt. Proper scoring dictates that it be notated straight, but the lilt may well be the preferred playing style. How would software cope with that? Or a swung beat? These are neither especially complicated, but the playing is not strictly in accord with the notation when both are correct.