A way of carrying multiple buckets of water at once. no idea how it'd work or how much it'd cost.

Maybe it could act just the same as a water bucket, but when you click to use it, it just decreases the level.

Maybe the player could walk slower while carrying it (speed dependant on how much liquid)

This is for the most part already addressed by the Diamond Buckets+ Forge mod. Looks like it's been around since MC 1.2.5 and updated for 1.6.2. It's not really widely known though. If you still would like to use this concept, that's a good place to start for ideas.

@mikeemoo Do we plan to use this concept?

How about just giving tanks the ability to scoop up liquids like a bucket?

New Item: 'Funnel'
Holding a funnel will work like a bucket but the liquid is placed in to a tank that is adjacent to the funnel.
So the tank must be in the hot bar to be used.

Thought about it. Nope.
If you want the advantage of carrying more than 1 bucket per slot, then fill a tank and break it. Convenience isn't free.

Closed on the case of balance. Fill a tank before you go. You CAN fill it with buckets.