I suggest dogs that can sniff out aliens instead of developing a motion tracking device.
1. Doable immediately (i.e. no new tech has to be developed by XCom) provided there are live aliens to train the dogs. Dog can be put into training cycle just like new soldiers.
2. Fits better into the time frame of the game from a technological perspective (late 70's).
3. Can detect stationary as well as moving aliens.
4. Can track aliens in addition to detecting stationary or moving aliens.
5. Have substantial melee capability.
6. Can be used for live capture instead of stun rod.
7. Come on, everyone wants to see a couple XCom dogs tear up those alien bastards.