I would think that it would be something that slows descent to a point where you don't take fall damage. It probably wont affect anything until you start falling. However it would require people to spend a lot of time trying to airshot/shoot the floating soldier. This would be less of an issue if the parachuting soldier was unable to reload instead of raining down constant rockets. If the soldier can reload while floating a mile in the air it will almost definitely be banned.
The problem I see is that the roamer has to do his job with two rockets since he will rarely have enough time for more against a good team, the parachute would almost guarantee 3 rockets. However it would be really funny seeing people get stalled in the air by hitscan and watching all the projectiles fly upwards.
Edit: Maybe it will have some kind of recharge on it like jarate or whatever? This would prevent it from letting people rocket jump around like idiots forever in pubs which valve cares more about. If this is the case it would probably be activated with right click since soldier doesn't use that, or taunt maybe idk.