So I was thinking about expertise dice, and I can't help but wonder if having a unique progression for each class would be better than having a single progression. Maybe they want the single progression to make multiclassing between martial classes have a 'common ground'?

But...

Each of the three classes currently available could easily end up with different progression tracks. The fighter's current track makes sense...a small number of large dice fits with the class. On the other hand, I think the rogue could stand to have more dice, but smaller sides.

If a fighter has 3d10, a rogue having 6d6 or so seems reasonable. The idea is that the fighter typically burns their dice on one or two 'big' maneuvers each round, while the rogue might use a couple die at a time, on 3+ 'small' maneuvers. The rogue's only big, blow-all-your-dice-at-once trick could be sneak attack, which usually can't be used consistently(in theory).

I could see the monk using a mix of Ki and expertise dice. Instead of getting X number of dice, what if they only advanced die size, and got Ki to spend instead of additional dice? The idea is, unlike a per-round dice count like a fighter, a monk has a per-encounter Ki count, and he can spend Ki on either his monk shenanigans or to roll however many expertise die he wants. Spend 2 Ki, roll 2 dice. Ideally, you'd want to design the monk's maneuvers such that he's encouraged to spend Ki as he needs it throughout an encounter, rather than blowing it all in the first round. More practically, some per-round(or per-maneuver) limitation on Ki spent might be a good idea. I could see 1/2 monk level rounded up being a good baseline on 'max-Ki-per-round'.