|That OSRIC cover always makes me think "Jazz hands!"|
There are elements I'd add from more recent systems, such as D&D5e's Advantage/Disadvantage system, and the much more intuitive ascending AC system with attendant combat bonus. There are other things that would need to be adjusted too, to get around absurdities like only thieves ever being able to even attempt to pick a lock or only rangers ever being able to have any chance of tracking.
I've seriously considered using D&D5e with all the "Old School" optional rules and restrictions in place to make it a bit less of a fantasy-superhero game, and that could conceivably work quite well for what I want, but I think it would tend to be confusing for the players who would have a whole bunch of expectations from other campaigns which would be doomed to disappointment, leading to resentment and muttering.
Anyway, it's just the germ of an idea at this stage.