start on my own or negotiate with the employer?
makecheck: Leaving the company to work on a competing product will sometimes violate any agreements you may have signed for this employer, so check those first.Usually the barrier to doing a parallel rewrite is staffing.  If the company barely has enough people now, sending people off on something else needs (in the management's view) a very good reason.  So you have to show how time will be saved, at least medium-term.Another potential barrier is testing.  If your product has a lousy set of regression tests and unit tests, forget it; until more tests are written, a rewrite is far too risky.  Basically, the test suite needs to prove what all the important current behaviors are, so that you can be very confident in replacing code.Is there something wrong with the code base?  Is it hard to maintain?  You'll probably have more success proposing changes if this is true.  For example, you can pull up the last 20 bugs/enhancements, and explain to your management exactly where all the time was spent on them; then show how your revised design would have made those changes more efficient.  If you have access to the time it takes customers to do things, you could have similar analysis; show how awkward or clunky something currently is for them, and how much better it would be if it worked differently.