I am running Grizzled Adventurers again this weekend (third time in five weeks!) and I'm strongly considering using the Shadowdark quickstart rules on my side of the DM's screen. I like the use of a single d20 resolution system, rolling high, advantage/disadvantage, etc.
At a glance, it looks like using the Grizzled Adventurers character playbook and standard OSR monsters will work out fine, if less deadly than RAW Shadowdark. (I run pretty dangerous dungeons in general, so it's not necessary, IMO, to make them harder.) I'm especially looking forward to no darkvision and the real time torches rule.
Shadowdark turns out to be just about exactly what I wanted but had not articulated to myself yet: a modern core, simplified to the complexity level of older versions. That would do most the job, but the way wizards and priests use up spell slots, or don’t, turns out to pretty well fix my decades-long problems with Gygaxian magic, without the complexities of 4e. So I’ve signed up.
I love the playbooks and subsystems each game includes. Even if I run Shadowdark as-is, I can definitely see using the Beyond the Wall setting creation system and the on-the-fly adventures in each system.