Great maul has some interesting properties, giving it both heavy advantages and disadvantages.
One of its disadvantages is its difficulty of use when solo. Even then, it has many things going for it: crushthrough, knockdown, etc.
However, I feel that its ability to be sheathed mitigates many of its disadvantages. Being sheathable is fine and dandy for most weapons, but when it helps you get in close range for unblockable attacks with a 1h+shield or top tier 2h/polearm, that's where I feel that it's a problem.
In my opinion, it should just be unsheathable. This won't affect dedicated users of this weapon at all. I could see the argument for 3 slots too, since it's a heavy boulder-like weapon.
I think if you decide to use a maul like this, you're dedicated to it just like a pike. It's hard to deny that it's one of the top support weapons, and I feel it's unfair to have an uber support and 1v1 weapon that you can switch between at will.