I'm curious about this, they sadly never got to play before Henin retired. I feel like the consistent depth on Azarenka's shots would be able to pin Henin back behind the baseline just enough, to prevent her from taking charge of the rallies and that Azarenka's return would put a lot of pressure on Henin's wonky serve. Azarenka would of course struggle on her own service games, but I think she is able to place her second serve well enough into the Henin BH which is Henin's weaker return stroke, much like she does to the Ivanovic BH to prevent Anci's hyperagressive FH return.
Comparing the parts of their game...
BH: equal, perhaps the edge for Azarenka as she is not limited in her return like Henin is because of her one-handedness (and height).
Serve: Peak Henin defo has the better serve, but in her last years her serve was extremely inconsistent so I would say they're mostly equal on a consistent basis.
Netplay: Henin is notorious for her good volleys so I would give the edge to her, but Azarenka is very underrated when it comes to her net skills.
Defense: Henin, her BH slice from a defensive position is extremely effective against player like Sharapova, who Azarenka is oftenly compared to as a weaker version. That said, Azarenka is way better at dealing this sort of junk than Sharapova thanks to her better movement and footwork (see Pova's and Aza's match against Stosur in YEC, Pova struggled immensely with her new BH slice while Azarenka cleaned up on them).
I would say Azarenka would be able to snatch one or maybe a couple of wins from her, similarly to her H2H with Clijsters, but in the end Henin would win most matches between the two of them. Thoughts? I really wanted this match to happen.