Following the Rocket Puppy Line:

0/1* Fires in a straight line (fire and forget)
2/3* Can be steered, but w/ poor maneuverability
4/5* Steers better

Cons
>Charge-only use (like Bombs)
>Player movement is slowed while charging
>Player cannot move while guiding a missile
>Range limited to slightly better than handguns/Construct lasers (auto-detonates after X amount of time)

Pros
High Damage
Bunker-busting mobs at range
Makes you a bad-ass

Notes: Players should be able to detonate a guided missile prematurely by tapping Attack once more upon firing. (Normally it would explode on a timer)

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Rockets already exist in the game in the form of Rocket Puppies, so part of the work is already done. It's only a question whether OOO wants to / can balance it for use by players.