I can’t think of any more system-neutral versions of the concept off hand.
I think, since the “plus” range doesn’t scale too far from 1 to 3, it probably isn’t too far off what most games can handle, although if you are playing a d20 derivative that doesn’t usually increase attack bonuses for spell attacks, you may want to avoid the foci in game.
Even the items that grant spells by expending charges shouldn’t be too hard to adapt, as long as the version of the game you are using has the equivalent of that spell.
It might get trickier with d20 variants that condense the level range to 1-10, at which point you need to look at if the game is expressing D&D level ranges from 1-20 as 1-10, or if it’s just intentionally cutting off power progression where D&D would usually evaluate “10th level” would be.
I’ll keep an eye out for any other versions of this concept for other systems, and thanks for reading the reviews!