Flash grenades sound reasonable
I like the idea too. I wonder what effect should the grenade have. Blindness? Paralysis? All the different conditions are really nice for tactics and they are really distinct: paralysis (the target looses a turn or two), panicked (the same, but likely to happen again), blindness (reduced accuracy), asleep (wakes as soon as attacked), confusion (semi-random movement), stasis (looses turns but immune to damage, not likely except by magic in an RPG:)), bleeding (looses health over subsequent turns), burning (the same but can spread), unconsious, under mind control, etc. Perhaps some weapons could also induce a condition?
melee weapons
I really like them. They are totally distinct tactically from ranged weapons and have their use when fighting inside buildings, when charging to defend a civilan/CEO/valuable equipment/etc., when fighting against enemies that have no ranged weapons/are out of ammunition/are temporarily disabled. They should be quite powerful and fast, to compete with, say, point-blank assault rifle in auto-mode for close combat (but the knife only has to compete with the pistol). I remember I usually killed most of the aliens in the final map of TD1 in maximal difficulty by standing next to them and hitting them repeatedly with autoshots from a shotgun or assault rifle (from afar I didn't do enough damage and so I risked retaliation). It would be more natual to slash them open with a sword several times under different angles...