1) For instance 10 gates under same hotkey. (Could perhaps also allow different types of buildings to allow for more unit hotkeys, and thus keeping a rather low limit as to how many units allowed under each hotkey, like in BW. 24 seems like a decent number for a maximum.)
2) Press build-unit hotkey once for each unit you want to produce. (so 5zzzzzzzzdd and you got 8 zealots and 2 darkoons in production)
3) It automatically cycles to next gate in the hotkey (no annoying forced tab needed but allowed if you want to skip production from that particular gate cause it's on the wrong part of the map, and then it skips to the next gate under same hotkey.
Once you've gone through them all, you're start from the first gate again, but producing more will add 1 to cue and switch to the next.
4) Of course you can still manually select a gate and produce normally from just one if you'd like to produce from an island gate for instance, or add several units in a production cue from a certain gate.
This solution offers the best between keeping strategic unit production choices for a mixed army and still allows UI simplification in terms of macro.
5) For zerg players, this could also be expanded upon by going to next hatchery instead of leaving off at larva or they'd have to manually tab to go to the next (since this would simplify things more for zerg than the other races, and they may still need some nerf to this possibly.. or perhaps see 6)
6) zerg hatcheries could be more expensive if not built in contact with existing creep, to reduce their ability to swallow a map, or each additional hatcherý could simply be 25 minerals more expensive etc etc..
7) Since you'd be spending a lot of off-base macroing, perhaps allow the hatcheries to display number of larvae available when hatchery is selected, so that you'd know if you would build 1 or 3 overlords. Again, for finer control you could still select individual larvae as before.
8) When you select a hatchery, upgrading should be the secondary option, not the other way around. So when you select a hatchery, available larvae are also selected.
Today, when you select a hatchery, you get the options of upgrading or researching as default instead of building (which is stupid, since you build units 99% of the time you select a hatchery, not upgrading from it).
9) Allow a hotkey of it's very own (customizable and separate from 1-10)for zerg that centers on a hatchery, and you can cycle through your hatcheries, since with a hotkey system like above, it will only help you produce more efficiently from them, not center on base.
To sum it up: MBS should not make the game play on it's own, but make the UI more intuitive, flexible, easy and accessible to new players and not unecessarily construed. I don't want to break my fingers going through 5-10 alternating pressing z or d for unit production when playing protoss for instance.
Add your own suggestions and comments on MBS and UI.