The rumbler is fun to use but lacks variety. At minimum there could be a second variant with grenades that explode on impact instead of after a delay. This would not obsolete adhesive charge because that blessing has a large damage boost against ogryns and monsters.
Not sure what you could do for a third variant. Maybe a rumbler with weaker grenades that stick to everything, not just ogryns and bigger.
I don’t think this would be a good idea, but another weapon could be a “rumbler” that shoots slugs. Give up the splash damage in exchange for maybe a few more shots, better handling/reload, and a touch more direct damage or armor penetration.
The “rumbler but shotgun” niche is already supported by the kickback. So that would make more sense as a kickback variant, the same idea but slug rounds instead of buckshot.
There is a little overlap with gauntlet but only in the sense that one of the rumblers is meant for single target precision shots. I’d configure the variants to be something like this:
Rumbler 1: Highest damage and splash, explodes on a delay, doesn’t stick without a blessing.
The default rumbler that we already have.
Rumbler 2: Medium damage and splash, explodes immediately on impact with anything.
Less damage than rumbler 1 but more precise since it doesn’t bounce. (but you also lose the ability to bounce around corners).
Rumbler 3: Lowest damage and splash, longest fuse, sticks to all enemies, big impact damage and headshot multiplier, big damage boost against stuck targets. Extra ammo.
Rumbler 3 is all about sticking enemies in the head, it has big single target damage but the long fuse time makes it less reactive and it doesn’t have as much crowd control. If you miss your shot the grenade won’t stick to the ground and might stick to the foot of an enemy who walks over it.