I would love to see the Grenade Launcher from Reach brought back.
I realize it was not popular, and they re-tooled it as the Sticky Detonator. I'm not against re-tooling it to make more people like to use it, but here's what I would have done (bear with me here, this may sound a little crazy). The Reach GL had 2 modes of fire, I would have eliminated the primary mode (trigger pull-and-release, 1-bounce detonate). I would keep the secondary mode (trigger pull-and-hold, detonate on release), so that no matter how short your trigger pull is, it detonates on release. Make it so you can easily suicide with it if you jerk the trigger. Think about the Rocket Launcher... it's easy to suicide with it if you don't use it smartly. People would learn to use that trigger pull smartly pretty quick, and I think they would enjoy it if made to do so. Casual players would learn to use it quicker. Losing the primary mode would make it more useful but more dangerous to the player at close range (again, like the rocket launcher).
Keep the EMP, keep the motion detector screen (which was present in different forms in the GL and the SD.)
I like the detonate-on-release because it becomes a skill-based 1-hit-kill weapon, surpassing everything other weapon in that regard except the sniper rifle (well... it was before H4 made sniping such a breeze). With the EMP it is also a skill-based vehicle punisher. People complain about plasma pistols in loadouts making hell for vehicles... I think the GL was a reasonable vehicle stopper in that the range was more limited and the timing was crucial. If you hit a vehicle with it, you deserved to wreck it.