For example, my read is that most of the people who are complaining about PCs and monsters being built on the same platform are really complaining about poorly written statblocks that are confusing and require references to other sources.
That is part of it, but the bigger issue is that customizing the monster is unnecessarily fiddly. Derived numbers are a pain. I did a lot of my 3E monsters in Word documents. When I copied a monster and scaled it or added a 3E template, I'd practically have to rewrite it. Then the effects of buffs on top of this in combat is even worse.
This takes times to grok. Plus, even if you aren't obsesive about it, and simply eyeball the numbers, you want to be in the ballpark of where it would have been had you done it the long way. Otherwise, you don't get the full benefit of the "challenge rating" or "encounter budget" systems. And bottom line, DMs builds a heck of a lot more custom monsters and encounters than players do characters.
So I don't think you get a compromise position with a 3E style that does mainly 3E stuff, only better.
I do think there is room for a sort of "compromise" that diverges from both 3E and 4E methods to cleanly emulate the the feel and results that are in the middle. The key to this is a careful selection of core abilities that are iconic for the creatures, but more expansive than what 4E did in this respect, and then supplemented by templates that carefully avoid derived values, and by monster "roles" that share some with player character "roles".
For example, take the simple goblin. You don't have one common ability to unify goblins. You pick two to four common abilities for the base goblin, and that's all he has. Let this set of abilities unite to really give the feel of goblins. Then you include some decent flavor and description text that fits those abilities. Goblins really stand out now when you read them, and they stand out in play consistent with that reading. So already we are sticking with a fairly simple stat block for the 4E crowd, but supplementing with more text and lore for the 3E guys.
Next, when you want to customize the goblins, you start pulling from those templates and roles. If you want a goblin shaman, you don't need a special set of abilities just for that. Instead, you go pull from the "controller or artillery" list and then when you really want him to stand out, add on direct from a shaman class. If he is also a werewolf or mad alchemist or any number of such things, grab a template. Now, the things that makes this goblin different from other goblins are things that you pulled from a semi-common pool. But his goblin abilities are still core to being a goblin. So the 4E crowd has a slightly more complex stat block (more abilities on average) but gets some enhanced flexibility in return, while the 3E crowd now gets some immersion hooks into those common abilities. (And you don't need to be perfect here for either side to be reasonably happy. You merely need to give them something to hang onto.)
And of course in the MM, you include a few examples of such things that people will want to use, fully fleshed out with the abilities repeated right there in the blocks. Same way with adventures. If you make your own and don't have software specifically for that, you'll have to cut and paste a bit in a word processing program, but that isn't really that big a chore once you get cranked up. Everytime you add a new type of goblin, your list gets more comprehensive. If a functional monster builder does this for you, so much the better--even if all you do is let it make a first pass and then copy it to your word processor.
In order for this to work, however, you have to really make abilities that don't depend on each or other parts of the listing with a lot of recalculation. That is a change for both 3E and 4E abilities. Take making a goblin "bruiser" for example. Let's say that is base goblin, but with some "brute" ability added. That brute ability doesn't need to have embedded numbers that have to be figured based on the goblin. Rather it needs to assume that the goblin is already using some core attack (as any monster would have) and the bruiser ability supplements this.
In 4E terms, you might have a rechargable or encounter ability that isn't used by itself, but supplements an at-will melee weapon attack. And do we really need to calculate to hit and damage for "brute smash" when all it does is "my club smash, +1d6+3 damage"? And if the ability does something totally different, like knock someone prone or daze them or whatever, then even better to not repeat the same damage. That way, when you tack it on a tougher goblin or a gnoll or a troll, it works exactly the same way, everytime.
