It's entirely possible to use QMK instead of XKB for your remapping purposes. But to do that, you need a programmable board or a QUICKIE device. See the BigBag. You cannot impement QMK as a OS component, unless you're a C programmer with a hell of too much time and skill on your hands.

Whether QMK is in fact all that superior to XKB is still debatable. They do different things, basically. For making multilevel mappings they have different tacks. QMK can only send keyboard key presses, not the symbols resulting from them. Therefore, you'll need both the OS keyboard mapping system and the power of QMK if you want both symbol-rich layers and fancy key stuff like Tap-or-Mod keys. On Windows, I can make the EPKL program do most of the things QMK does while also providing a plethora of symbols and strings, but it's hard to code in and I don't know of anything like that for Linux. It'd be possible to make but I'm not sure what tools exist for it.

I actually forgot about KMonad. Silly me. Yes, that's indeed the closest you'll likely come to the comfort of QMK on your Linux (or Mac or even Windows – although on Windows there's some decent competition as mentioned) system.

In XKB of course, it's easy to add all sorts of symbols, or even put in Uxxxx values for unicode symbols that don't have XKB names. But because kmonad (and QMK) only send keycodes, you can't send arbitrary unicode characters directly. 

Kmonad can use the linux compose character system to send characters though. For example,  the sequence of keys "compose 1 2" produces ½. So such characters can be typed using kmonad, but what's actually happening under the surface is Kmonad is sending three keycodes in quick succession which is converted into a single character by the OS.