There's been much to-do lately about the 5e playtest materials. I have been neglectful / busy, and as a result have not examined them. However, one thing that people keep going on about is their advantage / disadvantage mechanics, where if you have a significant advantage like flanking, you take the best of 2d20, and if you have a significant hindrance like being blinded, you take the worst of 2d20. While the application of this notion to skills is interesting, what it does most is remind me of an obscure little 3.x variant by the name of Codex Martialis.
Codex was probably the second system I read which favored an older-school grade of lethality (the first being Warheart), and it also was impressively well-researched historically. Hence, it springs to mind that there is some similarity here with ACKS, and I'm kind of wondering if they'd play well together. Handing out martial pool dice as a function of to-hit / save increases, much like class proficiencies, would work pretty well. Codex's weapon range system would also play nicely with the narrative style of combat ACKS seems geared towards. Finally, die pool allocation between attack and defense is very much in keeping with the calculated risks and resource management mentalities inherent in old-school dungeoncrawling.
That said, while such rules would be a fun experiment, for once I actually feel like not going through with it. ACKS' combat is gloriously fast and, while occasionally frustrating ("You miss the spider. The spider misses you. Repeat."), I think the constant fear of sudden death helps a lot. I'm actually starting to think that the "roll initiative every turn" rule is there to keep things unpredictable and dicey; random re-ordering every turn keeps people on the edge of their seat. Codex combat, on the other hand, makes things a bit more predictable, actually - the whole use of the dice pool is to remove uncertainty by either attacking many times, boosting your defenses, or going for one big surefire hit. In that sense, it runs counter to ACKS combat. Then there's the complexity factor, which would cause individual player turns to take longer, thereby making those turns where nobody hits that much worse.
Conclusions: Overall, ACKS and Codex would be a decent match if we were seeking somewhat more complex combat with similar lethality. However, for the time being, I think it'll remain a thought experiment.