Alright, this thing will obviously be heavy, and since the scotmann's skullcutter slows you down, this thing should slow you down even more, noticably at least.
Move speed -25% since the battle axe reduces by -15%
And since the axe gives a +20% damage bonus I think a 30% damage bonus would be fine.
Now, since you're probably as slow as a heavy by now, I think it's safe to say that charging CAN be aloud, despite the other recommended stats I saw for this, but I think you wouldn't be able to kill anything without a sheild to charge with, so I'm going to think that this warhammer will have an opposite effect the Claidheamh Mor has... Or how ever that under-used sword is spelt. So this weapon will reduce the distance you can travel while charging, so this will be a very good close quarters, indoors type of demoknight. Since the Claid-whatever gives an extra 0.5 seconds, this hammer I think would give a -0.5 decrease in charge duration.
Now, the damage buff is nice and all, but with this movement speed debuff, I think a passive bar on this weapon would do very nicely.
Since you can't take heads with a freakin' hammer, this bar will gain a certain percentage with every hit you land, (mini crits and crits add more percentage respectively if you have a buff banner soldier or a kritzkrieg medic helping you out.) And when this bar is full, you cannot use your sheild anymore. When you right click, instead of charging, you will go into a rage state, similar to crit cola and the buffalo steak, you will deal mini crits, be more vulnerable to damage, and give you a very good speed buff. While in this rage state you will gain a bloodlust, gaining some health, (I think as much as a halloween chocolate bar or the pill container would suffice) Every time you KILL someone. Also, if you have a pipe bomb launcher and the tide turner equipped, for example, while you have the rage activated you are stripped to this hammer only. (technically this passive is a buffalo steak and candy cane baby except a health kit doesn't drop, the health goes to you instantly on kill. If this is a little too OP for a passive I suppose a health kit could drop instead of going to you automatically)
I also thought if all this won't do, then this hammer would have a similar effect to the Natascha, slowing enemies on hit, literally breaking their bones each hit until death. Maybe the slowness can be stacked? Maybe this slowness on hit could replace the rage? Maybe since it's so heavy, Pyro's might not be able to airblast you as far as a regular demo? Maybe the bar is activated automatically like the soda popper when it's full? Just my thoughts.
But if I had to choose I would go with the rage bar.
So, to sum this all up:

Blue/Good stats:

30% damage bonus
When meter is full: Right click to go into rage.
While enraged, +45% move speed, hits will mini-crit, gain a small amount of health with every kill

Red/Bad stats:

-25% move speed on wearer
-0.5 charge duration
-20% slower firing rate (ubersaw speed)
Cannot use primary or secondary while enraged
More succeptable to damage while enraged
Cannot use charge when rage meter is full