I consider this
post by Ed Wischmeyer to be one of the best I've read on the subject of determining which functions earn their way onto the stick. Simpler is better in the long run. Also, in my construction blog there is an
entry that captures my thoughts on stick buttons for my RV-14A. I used Ed's logic to assign functions to the various buttons.
When you're musing over stick buttons, don't forget that they do not exist in a vacuum. Make sure you look at the system as a whole. Where do you want to position the flap switch? TO/GA (if included)? Think about how you're going to operate the aircraft. What are your flows? How are your hands positioned during takeoff, climb, cruse and landing? What functions do you need to perform in each of those phases of flight?
If you go through this process, you'll end up with a simple layout that is less prone to operator error (e.g. unintentionally bumping the starter button).
Good luck. I look forward to seeing what you decide.
__________________
Art Jackson
RV-14A Kit#140433, N393AJ Reserved
Completed:
Vertical Stab/Horizontal Stab 
Scrapped:
Rudder 
Working on:
Empennage (Elevator)
Construction log -
mykitlog.com/ajackson
Dues paid on 10 October 2018
Member of
EAA Chapter 92 (KCNO)
Pet peeve: "Lose" (rhymes with "booze") is the opposite of "find". "Loose" (rhymes with "juice") means "not tight".