I'd like a missile that slowly gets faster and has the potential to rocket across the chasm of Colosseum. Like "dropping a missile" in zero-G.
You fire it at roughly half the speed of a concussion missile, then it accelerates until it's going twice the speed of a Predator's maximum speed, leaving a distinctive smokey trail.
So it's not a Mercury missile (which is more like an enormous whistling bullet), but something that could still potentially be a sniping weapon with the giveaway being the rocket sound. The reticle is a crosshairs. It could have a slight zoom or "distant destination crosshairs" function as well if you hold down the trigger before release.
The explosion is about as deadly as a homer or a little more and the splash damage is higher. It could be distributed like a smart missile in terms of balance/availability.
The idea is that it will accelerate to its maximum speed quickly, but there is delay after pulling the trigger, as a fun and interesting balance mechanic.