I read through many of these posts and a lot of the stuff on the wish list starts to put the price in the 10K range very easily.
IMO Simple but effective things should rule.
Manual clutch engagement. Walker has been doing it for years and it works fine. No expensive electronic clutches and less needs for extra wiring
Belt driven- Blades and pumps. belts are easy to replace and while a hydro driven deck or a gear drive deck would be nice its $$$$$
Deck. make it simple to remove. IE like the walker ability swap out decks. in under 5 minutes
Seat- suspension seat. Its a back saver and much less fatiging. yes i adds to the cost.
Deck height- keep it manual with foot assist pedal- cheap, simple. nothing to break.
Deck- put some sort of an "easy jack" to drive up on for blade changes and such. Utize the front 1 1/4" reciver. see below
Controls- I like the idea of pilot controls, but like everything else you are adding complexity and potentially more moving parts.
OCDC- is a must. but i would go one step further and add an electric one. with a switch on the sticks. slight departure from my KISS principle
Muffler- Car quiet. if we can quite a 600 hp V-8 a 20+ hp motor should be easy.
fluids/filter-Make em all easy to drain/change. with no mess.
front casters-make so you can lock them in the forward direction. aids in hill holding ability along with the ted brakes.
hitch- 1 1/4" receiver style Front and back. No need to reinvent wheel.
Electronics- make some 12v plugs front and rear at the hitch for accessories (spreaders sprayers, ect)
Electronics- this is tough as mowers are a very harsh environment. Id go with fewer gauges and more along the lines of the tiny tach which is multi functions. many skid loaders have a display that does temp, hours, rpm, ect all you need is to push a button to access. for any alarms, all you need is a warning light and alarm.
electronics- IMO is a huge nemesis. all it takes in one loose wire or a bad safety switch and you are scratching your head. try to keep it as simple as OSHA will allow.
Fuel tanks- if you want to go removable. I like the fuel tank idea. but even better would be to incorporate some sort of braketry to allow portable fuel tanks already on the market, Blitz, eagle, ect to be used. Ie we get low on fuel just unhook the tank and plop the new one on.
Tires- if you go semi pnuematic, it will result in a harsh ride, and expensive replacement, and a heavier unit but flat tires area huge nemesis. and I;m not sure you want to the expensive route of suspension
Cup holder- eh. most of em you cant use as the drink will bounce out...
trash can- some sort of easily removable trash container that can be reached by the operator. A location for the a trash grabber to be mounted
Green- offer several engine options, propane, dsl, ect. they make catalytic conveters on small motorcycles. it would not be hard to fit one on a mower. be one step ahead of the green curve.
Noise- figure a way to get it quiet.
Color- Better is green keep it green
Name- Corriher 48 by Better
Parts- try not to reinvent wheel. use as many off the shelf components as you can. it will help keeps your costs down. but on the down side it gives folks the opportunity to buy their parts from someone else and not Better.