I use this, but I do a few other things as well that might mittigate this to some degree.
1) I have the characters start by distributing +6 total bonus across their abilities, maximum to a single ability +4, able to trade out up to -2 from other abilities. By this I mean the following: Str +0, Dex +0, Con +0, Int +0, Wis +0, Cha +0 can become: Str +1, Dex +1, Con +1, Int +1, Wis +1, Cha +1 (effectively all 12s), or it may become: Str +3, Dex +2, Con +2, Int +1, Wis -1, Cha -1 (effectively 16, 14, 14, 12, 8, 8). Thus the total modifiers must equal to 6, and total negatives cannot exceed -2. To deal with feats I change the requirement from an odd ability score to its equivalent ability modifier.
The result of the above is that every fourth level an ability
modifier is increased by +1. This is akin to your initial post (of changing +1 to an ability score to +2 to an ability score). It also deals with the issue of odd numbered scores - simply because there are none in my houseruled system.
I've been trying to shy away from ability scores in favor of modifiers for a while now, chipping away at anything that uses a score instead of a modifier on a case by case basis as it comes up. There are still a few things I haven't changed over yet, but they tend to be minor things that are not encountered too often (at least in my games).
2) I use a lower magic world - by delaying casting by a couple levels (see spoiler below). This, combined with the fact that spells that affect ability scores / modifiers and spells that raise dead have been moved up two levels (see spoiler below), means that Bear's Strength, etc is not available until around nineth caster level. I do not place a limit on Brew Potion caster level, although I do use rules for potions going bad over time, random results for mixed potions or drinking one while another is in effect, etc. Generally the more powerful the potion, the worse the result if it goes bad or is mixed.
Just as the potion of Bear's Strength is now notably more expensive due to these changes (1800 gp instead of 300 gp), so too have I raised the price on ability increasing items - which are already less common due to the raised caster level and spell level. The cost was tripled, actually, such that an ability +2 item [ie: a +1 modifier to the ability] costs 24k gp, an ability +4 item [+2 mod] costs 48k gp, and an ability +6 item [+3 mod] costs 108k gp. Thus even a 20th level character would find it unwieldly expensive to have a +6 item in more than two or three abilities, considering character wealth per level.
The final result of this is that ability increases by means other than leveling are virtually unknown until mid-levels and uncommon until high levels, making the increases via leveling (and class abilities such as Rage) far more important (as they should be, I think).
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True Resurrection . . Does not exist.
Resurrection. . . . . Level 9, not 7
Ability, Mass . . . . Level 8, not 6
Raise Dead. . . . . . Level 7, not 5
Ability, Singular . . Level 4, not 2
Caster level Spells Per Day Per Level Progression
Lv 0 1 2 3 4 5 6 7 8 9
01 1 - - - - - - - - -
02 2 - - - - - - - - -
03 2 1 - - - - - - - -
04 3 2 - - - - - - - -
05 3 2 1 - - - - - - -
06 3 3 2 - - - - - - -
07 4 3 2 1 - - - - - -
08 4 3 3 2 - - - - - -
09 4 4 3 2 1 - - - - -
10 4 4 3 3 2 - - - - -
11 5 4 4 3 2 1 - - - -
12 5 4 4 3 3 2 - - - -
13 5 5 4 4 3 2 1 - - -
14 5 5 4 4 3 3 2 - - -
15 5 5 5 4 4 3 2 1 - -
16 6 5 5 4 4 3 3 2 - -
17 6 5 5 5 4 4 3 2 1 -
18 6 6 5 5 4 4 3 3 2 -
19 6 6 5 5 5 4 4 3 2 1
20 6 6 6 5 5 4 4 3 3 2
I do grant the caster classes some bonuses to make up for this. The Sorcerer gains a Bloodline (UA), the Wizard gains Lore and another two skill points only useable in Knowledge and Language skills, and the Cleric gains Lore (and Lay on Hands instead of Spontaneous Casting). I never really saw a need to grant the Druid anything, although with the relatively recent changes to their Wildshape I may find that necessary now.
I view 1st level as the apprentice level and second level as the journeyman level. Third through fifth are master levels, while sixth and seventh are the grandmaster levels - at least as far as NPCs are concerned. Most games start out at with the PCs at third level, so that they already have some backstory, some specials, etc. This also allows a starting character to have traits of more than one class without the odd Apprentice levels suggested in the RAW. Note that most NPCs are Experts 2 to 4. Commoner is only used for serfs, slaves, etc.
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However, if you intend to use +2 to an ability every 4 levels in a
normal game, then I must warn against the certainty of increased power of the characters. In a lower magic game - even one only mildly so rather than my version - this could be useful to counterbalance the lack of magic. Indeed, it could also be used in a lower wealth game to counter balance the lack of magic items (due to lack of funds to purchase such). But in a normal game - particularly a higher magic game - this could quickly result in phenomenal characters. A wizard, for instance, could start with Int 18 and by level 20 have Int 40 (18 +10 for leveling +6 from wishes/tomes, +6 from a magic item). This is, one will admit, well beyond what is typically expected for a level 20 character. Indeed, even a level 12 character, for example, could easily have an Int nearing 30.