I think it should have an effect: When an enemy Scout is able to deflect back the Tennis Ball launched from the weapon

the bounciness of projectiles shud be a downside

i think this should be a good combination for this weapon

shoots tennis balls
+ stuns enemy for 2 second from 3 meters when hitting him with the ball (enemys will get bonk effect for 1 second when get hit from 6 meters)
+ shoots 25% further
+ tennis balls will bounce when hitting the ground
+ when enemy is stunned, 100% chance to get crit
- 25% explosion radius
- 50% damage
- 25% damage to buildings
No random crits!

good. only thing i can say is good.

(+,-) holding down primary fire will decrease the arc of the grenade
(+,-) very bouncy, after every bounce the knockback will increase
(+) grenade can still do direct hit after it hits a surface.
(-) if hit by own grenade you will have an effect like concusion grenades from tfc for few sec.
(-) 10 less damage

not direclly related to the weapon, but I think the demoman should have a tennis racket that can reflect certain projectiles called the murray maul but that's just my opinion

Moo-ray Mound Maker:

+ 60% quicker firing rate and reload speed
+ Crits cause 3 seconds of bonk on hit
+ The last shot in a clip always crits

- Crits do no damage
- No random crits