I think as a core list I'd want :
- one piece action
- floating barrel (not too thin - maybe 15mm or more) threaded for silencer/stripper
- direct pellet feed
- transfer port not inside barrel
- No AT fitted
Accepting that not everyone has the same requirements and will almost certainly want to tailor the gun for their own preferences, instead of trying to build one gun that suits everyone, why not offer a componentised gun.
So... offer it in an action only option (if you want to fit your own custom stock, there's no need to have to buy one with a standard stock and sell it on when you fit your custom one). Make the action so that it could fit in an S400/ProTarget/Ev2 stock without too much modification - so that people can use existing stocks if they want to.
Offer drop in options that you can upgrade/change later on.
Different cylinder options could include - standard, long, steel/titanium
Different reg options (easily swapped out by screwing / unscrewing from cylinder) - no reg, standard reg, match reg
Different trigger options (make the trigger mechanism a contained drop in component) - standard, match
Fully adjustable trigger blade with an easy option for swapping out the blade and perhaps options for an offset blade.
Offer some stock options for those that don't want a custom stock - hunter, junior, target etc
Offer different butt pad options - standard pad, adjustable pad, HFT hook, FT hook
Offer different bolt on rail options - dovetail, weaver, drop compensated raised rail etc.
Offer different barrel options - long, short, match grade, different twist rates, calibres etc.
All the options should be available at reasonable prices so that the gun can be upgraded later on without getting mugged on the price.
You'd have a gun that suited everyone then.