All metal construction for durability. No plastic crap.
Design to have grip portion removable, leaving the mounting ring for the grip exposed, so one can turn it into a thumb (or half grip) throttle if desired. (since it's unlikely they'd go for making those versions, and they'd sell more of them if people can use them for other than full-grip without using a hacksaw). Easy to justify because grips can become damaged in laydowns/etc. and being able to replace just that part or at least take it off easily to rework or re-cover it would be very useful.
Waterproof. Pot the insides to seal them.
Adjustable "range", to adjust the feel of the throttle for the particular application. This can be done with a couple of tiny trimpots for the top and bottom range, simply wired in series with the 5v and ground lines to the sensor (or pot) inside. A couple of rubber caps (like the Meanwell HLG series uses) can fill the access holes to these for weatherproofing.
Vectrix-like regen capability, so you reverse twist from the "off" position to brake. This will have a completely separate sensor and output, with it's own trimpots to adjust it's range/feel.
Note that it is necessary to have the trimpots, as not all controllers have ability to adjust their input ranges, and certain situations, especially mounting positions, may require a different feel or movement range. WOuld also make it usable by someone with limited wrist/etc motoin range, where a regular unadjustable throttle would not be.
There's a bunch of threads in this list
https://endless-sphere.com/forums/search.php?keywords=throttle&terms=all&author=&sc=1&sf=titleonly&sr=topics&sk=t&sd=d&st=0&ch=300&t=0&submit=Search
that discuss problems with various throttles, and solutions, that will help you, but you'll have to look thru them to find the information.
