Only started playing any Starcraft upon the retail release of SC2. While I love it, one thing I can't get over is how needlessly enormous the UI is. I have a 26" monitor and I see no reason why it needs to take up almost 1/3 of the vertical screen. 
 
Everything but the mini-map could be displayed in 100-200 pixels max. So will there ever be UI mods to do so?

I actually really want that too. It does take up a lot of space.

Not that it helps at all, but you can turn the UI off when you make a custom game.

You can make custom UI's in the map editor for user made maps. Having UI mods for matchmaking will probably never happen. 
I think Blizzard learned a lesson from WoW Arena addons.  

you prob won't ever be able to. It's kept to not give people a slight advantage. If your running a higher resolution you will have a bigger advantage since u will be able to see more of the screen at once. Thats why no matter how high your resolution the ratios of teh screens will be the same, they just will be at a higher resolution

Weird. Okay well if there's never mods, it would be nice if at least Blizzard would give an alternative mini-nav option. I wager 99% of people in ranked matchmaking would use it.

How exactly would all command buttons, group buttons, and selected units fit into 100-200 pixels?

How big are the icons on your desktop? They don't need to be 80x80 with huge border surrounds. They could be 20x20 max. 
 
Design talk yeah!

Never for match making. Certainly for custom games.

Knowing Blizzard I'd say never, they do love their sacred cows.

Considering the majority of the high-level competitive players have been playing Starcraft 1 for years and are used to the UI, I don't think many of the high level players care about reducing the UI. If it were an option, sure, but I doubt they're clamoring for it.
 
And it does come down to balance. Blizzard isn't going to allow mods in ranked competitive games for Starcraft.