As a start, I am modifying current Mugician code to be "Mugician Pro". I got some issue that's keeping the store version of Mugician from cleanly installing "Mugician Pro" at the same time.
The first thing I am adding is some FM synthesis capability - to give a much warmer and tweakable underlying tone before it mashes through the distortion. Since space is so scarce, I am taking the whole bottom row for sliders. Interestingly, no range is lost since the leftmost item on first row above the controls is already the lowest note. You only give up having an alternate fingering for the bottom 5 bass notes in that case; a tradeoff I could see coming.
Anyway, the plan is to at least put up Mugician Pro as a bug-fix branch of the free version with enough new features as would be commonly useful. I do plan on keeping this instrument simple.
I am not going to get into the game of trying to entice people to buy with more complexity. Every wierd new feature will only be used by a small number of purchasers, and I will end up with a big ball of goo trying to rope a lot of people by catering to all of them. You may note that there is not a single letter of text in this app. When I really have to put that first one on, I have failed in simplicity.