13th age is a D&D-style TTRPG written by some of the developers who made 4th edition. So it’s very, very D&D like with a much more solid core system backing it up. They just finished a kickstarter for their 2nd edition a little while ago too.
That all sounds like way too much work. You’re basically re-designing the entire game at this point.
IMO, your time would be better spent finding a system that already does all that. They do exist!
I would suggest maybe taking a look at 13th age, or some of the renaissance games like WWN and the like.
My personal thought was to remove counterspell from all non-wizard spell lists and turn it into a wizard-only spell. That alone…