While I do really like 13th Age [...] it doesn't really have the tactical crunch that I think a lot of 4E players want.
Only because it doesn't.
I'm being flippant but also serious.
13th Age has a
chassis for creating, oh, I dunno, let's call them "Encounter Powers" and "Daily Powers" that
could have as much tactical crunch as you want.
By default, 13th Age powers are relatively low on the tactical crunch scale (by no means at 0, though!). But you could dial that up as strong as you want and it would still be recognizably a 13th Age power
in format. Which would also make it weirdly very close to, oh, I dunno, the 4th edition of the world's most popular role-playing game.
I know it would be a ton of work (as was previously mentioned earlier), but I think a straight retroclone of 4E
How would you do that, though, given the constraints of the 4E GSL?
I think it would be much easier to take an OGL game like, oh, I dunno, let's call it the Baker's Dozen Time Epoch which is already
close to 4e in spirit, and then hack at it until it more resembled 4e. Than it would be to work within the confines of the GSL.