I started roleplaying in a Dark Sun inspired game and then began playing D&D with 4th ed and then Pathfinder. The magic items being baked into the game has been quite jarring for me and while 4th ed eventually lessened their necessity it's always been diifficult.
I finally got a game where no magic items were possible in 5th ed, which revealed I like my magic items, just not in the quantity that 4th ed and Pathfinder has them.
Here's my effort at introducing a houserule that mimics 4th Ed's "inherent bonuses":
Starting at level 2: +1 AC per level
+1 attack/saves at level 3,6,9,12,15,18
+1 to two ability scores at: 2,4,6,8,10,12,14,16,18,20 (replaces standard progression)
Weapon attacks count as magical at level 3.
Weapon attacks count as cold iron/silver at level 6.
Weapon Attacks count as adamantine at level 9 and when you full attack you get 1 extra attack at your highest BAB. This does not stack with haste or similar spells.
Weapon attacks overcome alignment based DR at level 12.
WBL at level 20 should be 390,000 gp with roughly a third of that spent on consumables (can be higher for pure casters).
My reasoning for these advancements uses this non-optimised but functional purchase of magic items for a fighter:
50,000 amulet
5,000 ioun stone of AC
54,000 second ability score booster (AC)
50,000 ring
25,000 cloak
12,000 boots
36,000 belt
137,000 manual
27,000 manual AC +1
50,000 weapon
30,000 +1 saves and attacks
Sub-total: 489,000gp
Actual items:
25,000 +1 enhancement weapon
11,000 +1 Armour enhancement
44,000 ring
8,000 extra dimensional storage
7,900 skill boosters
45,000 other
8,000 mundane gear
132,000 consumables
Summary:
AC: +16
Saves: +7
Extra attack
Attack: +6
Ability score boosts: 16+5
Does this seem to create a baseline advancement that would keep PCs on par with monsters? Obviously optimization opportunities will be greatly reduced, but as long as their kept on par that's the goal.
I've run with something similar. All but one player liked it, and he was strange anyway.
Here's what I've leaned:
It's important to track DR and resistances. Players had to fight a wererat, and the lack of silver weapons turned it into a slugfest. Effectively raised the CR considerably.
You need a spellcaster option. Maybe it's not as good for balance reasons, but a wizard with an adamantine dagger is just as useless as a wizard with a steel dagger.
That AC bonus seems too good. At first level, it's fine, but +10 ac at level 10 will make them unstoppable, especially if their dex or con is going up, and everyone will pick one of those as their second stat.
Here's the system we used:
http://www.enworld.org/forum/showthread.php?298121-Cutting-Down-the-3-5-Chr istmas-Tree-Decoupling-Wealth-from-Combat-Power
It's important to track DR and resistances. Players had to fight a wererat, and the lack of silver weapons turned it into a slugfest. Effectively raised the CR considerably.
Handing out non magical special material weapons would handle that without drastically overpowering the character against enemies who don't need that material to overcome DR. Silver sheen and similar consumables can also help. It might not cut down on the golf-bag effect but does help keep things low magic.
You need a spellcaster option. Maybe it's not as good for balance reasons, but a wizard with an adamantine dagger is just as useless as a wizard with a steel dagger.
As you point out giving them the weapon bonuses does no harm and you could easily give them 83,000gp worth of more magical items to help balance them out.
That AC bonus seems too good. At first level, it's fine, but +10 ac at level 10 will make them unstoppable, especially if their dex or con is going up, and everyone will pick one of those as their second stat.
I was worried about that, although it seems a flaw inherent to Pathfinder. +5 armour +5 amulet of natural armour and +5 ring of protection quickly adds up to +15 AC without using boosters to dex. Also your system seems to grant +23 to AC (adding all prowess and AC boosts 2nd ability score boosters). In mine you'll get +24. Is the main difference that you've loaded them onto higher levels rather than an even distribution?
Also a CR 10 creature gets an attack bonus that's +16/+12 higher than CR 1. Assuming core fighter in full plate their AC would have increased by +15. Other characters would only be at +11. It seems to be keeping pace rather than breaking the game. Would actual play reveal something different? Is there something I'm overlooking?
All of these bonuses would also technically stack with such spells as shield of faith, bull's strength/eagle's splendor. Would this be a concern? Or does the opportunity cost self regulate?