I don’t know how easy that will be to pull off for every character design.
Probably fairly easy. If you look across every variation of TMNT, most characters, and certainly the important ones, have elements to their design that repeat across all iterations. The changes are stylistic. So it's actually a lot like Marvel Legends where you have to distill those elements down to a non-stylized action figure.
An 'ML-style' TMNT line would just need to choose which parts of which versions of those characters it wants to utilize. The hard part is really just being spoiled for choice.
There isn’t always one iconic look for each character that will satisfy everyone.
That's absolutely true, but it's also true of every other TMNT line ever made.
A good example here is Mezco. They're doing their own version of TMNT, kind of doing what I mentioned above - pulling from the different sources and making a singular version of each character that is very clearly that character but doesn't owe its entire design to one specific version of that character. Those figures have been wildly popular and even people that cannot afford them seem to think they're excellent designs.
No reason Mattel can't do basically the same thing. It'll never satisfy everyone, and that shouldn't be the goal or they fail before they even get started.
I don’t know how great the designers at Mattel are at making a universal look.
It definitely depends on who they put in charge of it. But again, that's true of anything.
but with NECA having covered SO MANY of the bases, so many different specific styles, I don’t know what ground hasn’t been covered nor how creative Mattel can be.
The 'unified line' is literally the ground that hasn't been covered.
When it came out that NECA was going to do stuff in the cartoon line that hadn't actually been in the cartoon at all, but just re-interpreted designs into 'what if this had been in the cartoon?' - people went nuts for it. It was a GREAT idea. I would argue there is a hell of an appetite among collectors for a Turtles line that can bring in everything and do everything, all in a style that fits together. And if those figures can ALSO be displayed with Marvel Legends Daredevil? C'mon. License to print money.
And I guarantee you that Mattel is aware of that possibility, as was Hasbro.