I never really liked how saving throws worked in D&D. The concept of a roll to avoid some horrible fate seemed anti-climatic and antithetical to play, but that would be the topic of a different post. The mechanics themselves are rather stagnant.
At lower levels, saving throws are pretty poor and you don't really expect to make a save in most cases. At higher levels, saving throws can get so good that they can be meaningless. I'd rather have a mechanic that is interesting at all times. 3rd edition used scaling saves, but that really just devolved into the edition's min/maxing attitude; saves became binary, either you maxed out your save or DC or you didn't. Not too interesting there either.
The simple solution is to tie saves to stats, which are rather static. Roll a D20 and if you roll equal to or under the stat in question, you save. Saves stay relevant at all levels and the classes keep their unique quirks (thieves dodging fireballs, Clerics resisting mental effects). For NPCs/mobs without stats, we can make a quick correlation to HD. Maybe assume every stat is average (9), plus some fraction of their HD (half? third?). You will still have the monsters usual resistances/immunities (undead to mental effects) to guide any tweaks. Either way, saves for monsters are mostly "did he die yet or not" and are one more roll in combat; I'm not concerned with having a strict mechanic there.
In the end, stat-based saves makes sense with a nod towards realism. Hell, I am just as susceptible to my wife's ziti today as I was 8 years ago; my willpower hasn't really changed through my adult years. Soldiers get better at fighting, but they don't become wiser or more hardy with age. If you throw in the age effects on stats, you get a small movement in those stats to reflect aging, and that small tweak seems perfect enough.
Yea, nothing monumental.