I do like the idea of tying the DC into the spell level and I've tinkered with DC = 8 plus twice the level and DC = 10 plus the level (both still incorporating half damage, whichever is higher, etc.).
Actually, the only two casting-types that have CON save proficiency are the Eldritch Knight and the Sorcerer. Which means anyone else started off as either a Barbarian or non-EK fighter and MCed into a spell caster or would need Resilient to get CON save proficiency. I am certain that is why more characters default to War Caster; they already likely want to have their hands full if front-liners.
We've never had a single played take Skilled or Actor, but Ritual Caster, Spell Sniper, and Keen Mind once or maybe twice. Of course, we've only ever had one character with GWM and one with Sharpshooter (both by the same player, shocking huh?

).
We have some take ASI's since in some cases the increased modifier is more useful all-around that a feat. It isn't common, but happens often enough.