What is genius, James, the hf audio one? I want a bang!
For me, light weight and not sharp is essential - to be able to throw it properly without worrying about hurting people.
It should be modular so that the shells can be replaced to go with various loadouts.
The guts should be modular also so that the same detonator spring gubbins can have something else added/removed simply to change between impact or timed detonation.
It would be handy if the same mech could be used for timed or impact with just a twist of some rugged bit, like the position of the spoon relative to the body say.
Variable timer would be nice, but not really essential.
The guts should also be simple to repair when worn out - whichever sear/spring/whatever wear out should be available as a spare.
I'm not really bothered about 9mm, especially if it needs extra weight to prevent it jumping in a way which could hurt somebody when it goes off, but if it was possible to create a lightweight recoil absorbing mech so that it was louder, that would be ideal. But .209 primers are ok.
A built in screamer a la key finder fob would be nice, since they are lightweight, to aid finding it in dense bushes and make stealing it more difficult.