Durability: we'd like to see a gas system that offers the realism of heavily actuating parts that doesn't beat up on itself ...high grade steel and extremely high grade polymers would be a good start ...maybe carbon fibre?
Air seal and hop-up: the two most important factors in BB trajectory. Allow for a large and extremely aggressive bucking system that easy to work on. Also a perfectly fit together parts and simple system that capitalizes on excellent air seal. Avoid AEG or old hop designs. Check out Systema's for a few ideas.
FPS valve: a no brainer would be an FPS control valve, so one doesn't need to be installed aftermarket.
Robust internal system: look towards the Systema PTW. Everything clicks together "perfectly". Moving bolt, hop-up system, etc.
Magazines: must have a clever seal system, and be reliable. Make them easy to clean, rebuild, etc; and not to wear out easily.
Gas system: make sure the gas system can handle everything from CO2, to green gas, and all heat tolerances and variables and still fire flawlessly.
Upgrade ability: a good gun is easy to disassemble, and doesn't contain any cheap or useless parts. It's nice to be able to quickly do a barrel swap or switch out the bucking.
Cost: cost will reflect the build, but I'd rather pay $1000 or more for a ridiculously good build than buying something where the company tried to save money.
Performance: if your going to bother making it durable, dependable, and reliable, them it must shoot outstanding as well.
Bells abd whistles: RA-Tech has made some good upgrades for existing systems, check out some of their ideas. Look into things like perfectly milled stainless steel inner barrels, etc.
ďREALITY IS LIKE A STONE. TO MANY ITíS HARD AND COLD, THEY CANíT HUG IT OR EAT IT, IT ONLY FRUSTERATES THEM AND DOESNíT DO THEM MUCH GOOD. TO OTHERS ITíS STRONG AND DEPENDABLE, YOU CAN BUILD WITH IT, BUILD UPON IT, OR WORK WITH IT, ALSO USE IT TO SMASH PEOPLE IN THE FACE.Ē