I prefer a modest increase, if any.
I would do something like the 1e Unearthed Arcana rules for cavaliers but make it universal.
Every character starts with a stat and a percentile die roll. Every level, roll 2d10 and add to the percentile.
Cap it at racial maximum.
Ding!
This is almost exactly what we've been doing for ages, except the percentile increments can take you beyond your racial maximum. At roll-up you can choose two or three stats to advance via percentiles, of which one must be your "primary" stat e.g. Str for Fighters, Int for Wizard types, etc.; if you choose two they go up a bit faster than if you choose three*. But, some things to consider:
- this works better in a game with fewer levels
- this interacts VERY badly with 1e 18.xx percentile Str scores; we ended up breaking the various 18.xx's into their own whole numbers (thus 18.41 became 19 etc. up to 18.00 becoming 24) and jumped Hill Giants that used to be 19 up to 25
- it is really easy to speed up or slow down the stat advances by tweaking what dice are rolled for increase at each level. The 1e Cavalier used 2d10 per level above 1st; if you want faster increase you could use 4d6 or even 4d8, for slower use 2d6, and so on
* - in my current game if you choose to advance two stats the primary gets 3d8 per level after 1st and the secondary 2d6; if you have three advancing the primary gets 2d8 and the other two get 1d10 each
And yes, I too prefer very modest by-level stat increases. I don't mind them coming from one-shot magics such as those found in some classic modules, and I've mostly made wishes almost a random interrupt rather than something you can just go and buy or know where to find; most of the time people don't even know they have a wish available (e.g. from a Luckblade) until they say "I wish..." at some point and it happens** - by which time it's too late! I can't remember the last time - if ever - a wish was used in one of my games to increase a stat.
** - and even if there are no outstanding wishes, every time I hear someone say "I wish ..." at the table I roll some dice anyway and say "no", just to keep 'em guessing.
Lan-"brought half a party back to life with a wish while field-testing a magic item"-efan