1) The method used to detect a nearby enemy does not check to make sure he is in the line of sight (e.g. blowing it up from the other side of a thin wall.) This can easily be fixed using trap_Trace() or some other quicker function I am not aware of.
There is a G_IsVisible() or something that will do that for you.
2) If you drop a grenade on a human or alien head, and the human or alien moves away before it activates, the grenade will float (similar to Goldeneye 64's Floating Remote Mines). I am not sure how to fix this.
Check the ent->r.contents of whatever you hit with the grenade, if it is CONTENTS_SOLID, stick to it, otherwise fall with gravity or immediately explode.
I really like the idea to make it only buildable from the advanced construction kit, but if I do that it will not work without having pre-configured your client to allow downloads, so I think for now I will leave it like this.
Want to know something funny? Up to revision 451, Tremulous had constructible "float mines
". They were ditched a long with the bank. Too much Gloom resemblance for the devs?