I'm doing this in my current game
In an effort to move emphasis from magic items to characters, and with the help of ENWorld posters, I developed the following system to replace stat boosting magic items, both of the gauntlets of ogre power and tome variety. Stat boosting spells like bull's strength are allowed, but the overlap with the 'natural' enhancement bonus just as they would with magic items. Campaign is at sixth level so I can't actually vouch for its success, but I'm very confident.
Code:
Level Total +Enh at Level Inherent
Enhance Level Bonus Bonus
Bonus
1
2
3
4 1
5
6
7 1 1
8 2 1 1
9 3 1
10 4 1
11 5 1
12 6 1 1
13 6
14 7 1
15 8 1 1
16 9 1 1 1
17 11 2 1
18 13 2 1
19 ? ? 1
20 ? ? 1
Max Enhancement on a single attribute of '+3' through 11th
Max Enhancement on a single attribute of '+4' from 12th to 13th
Max Enhancement on a single attribute of '+5' from 14th to 14th
Max Enhancement on a single attribute of '+6' from 15th - 20th
So, at 7th level the characters get their first enhancement point. Also, they get inherent points at 4,8,12... as normal and to represent the tomes/manuals, give out another inherent point at 15,16,17,18 and 19th level.
Rationale: I used the Bastion of Broken Souls WoTC module and my own experience with PC's up to 12th level as a starting point. I gave the majority of consideration to the no-magic-item-greater than 25% of wealth standard. I left 19 and 20 blank because I didn't know what to do and no campaign I play will ever get that high.