I just noticed that in this season in one matchup I've twice the win-rate vs another. I suppose that is an extreme case, but surely most people feel quite differently confident in different matchups.
Current MMR aims for 50% win-rate altogether, with the allowing for variance in win-rates among the matchups - but the variance may be quite high. MMR per opponent's race would aim to stabilize at 50% for each matchup and thus allowing for more fun games.
The overall ladder MMR could simply be an average of these 3 MMRs.
This would allow for closer and better games no matter what the current balance is. Even when the game is well balanced for the pro players, there have been large disparities in win-rates for lower skill levels. With separate MMRs per matchup, this problem would be solved.
This may even help with ladder anxiety, not having the mindset that the player is already in a disadvantage before the game starts when facing the race with lowest win-rate against.