(Oooh, first thread! 
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There is a lot about Beyond the Wall I can’t wait to try: The communal character and setting creation, its nifty Scenario packs and so on.
What I struggle with when reading it is some of its OSR/D&D mechanics. The basic resolution methods are fine on their own, but the fact that there are 2-3 of them? Less so. Sometimes it’s roll-under, sometimes not, sometimes attributes matter, sometimes they don’t. These small inconsistencies rub me the wrong way and so I thought of ways to hack things a bit.
So far, my idea is to liberally take BtWs chargen (+Village-gen, etc.) and bolt it to something like Dungeon World/World of Dungeons 2d6+attribute modifier -> 6-/7-9/10+ mechanic, while retaining most of the mechanical bits around it. So there would still be spells/cantrips/rituals, you’d roll them when appropriate, damage and hit points could stay as is, and so on.
So far my idle musings have identified a few issues with this idea and I’d love to get some input/ideas here.
- Modifiers:
BtW uses modifiers a lot, especially in its magic system, where they are used to allow Cantrips to do more for increased difficulty or buff other characters with stuff like Bless. Since 2d6+Stat has a much smaller range and weighted probabilities its more sensitive to both bonuses and penalties and something like a +/-1 has a much bigger influence than a +2 on a d20.
Maybe something like advantage/disadvantage could work better here, but dunno? 
- Skills:
In BtW, skills are a thing. The skills you get matter a lot in shaping your character and sometimes you can get the same skill multiple time, showing that you’re REALLY good at that and providing stacking bonuses.
PbtA doesn’t generally do skills, tho. Either they are absent (Or codified on a different level as Moves.) or provide non-numeric advantages, like the “Can’t fail even on a Miss, just gets a bigger consequence” of World of Dungeons, or “just” matter for narrative positioning. (Allowing you to do things unskilled people couldn’t do.) When they appear, they tend to be binary, ie. you either have a skill or not.
Since I want to keep the chargen as much as it is in BtW as possible, I’d prefer not to change the fact that there are skills, that they matter mechanically and that they can poentially stack, but I’m struggling with how to keep that and use the simple PbtA-style resolution mechanic.
Maybe different skill levels could give different Bonuses like “Roll with Advantage” or “Can’t Fail on a Miss”, but it all seems kinda meh.
- Combat, BAB and the Fighter
The Fighter’s thing is he’s good at combat and that’s def. something I want to keep, but that means figuring out whether I want to keep BtWs combat as with stuff like BAB (and if I don’t whether I should keep the Fighter something to compensate/highlight its “Better in Combat” nature) or if I should change it to be closer to PbtA combat. (Less strict initiative rules/more fluid order of events, armor as damage reduction, etc.)
Well, those are my ideas and issues so far. Has anybody else thought about doing something similar? Any ideas or comments are appreciated. 
M