I voted "fairly low".
However, aas others have stated or hinted at, a short section of suggestions in the DMG would probably work best.
"For traditional/Classic/Adventurer game play cap scores (without magical assistance) at 18."
"For expanded/Legacy/Expert game play cap scores (with or without magical assistance) at 25."
"For epic/Legendary/Master game play (with or without magical assistance) there should be no cap."
But then, this runs into a problem with the stated desire that players using different modules/optional systems should/could sit down and play at the same table. If my "Conan-esque" fighter is maxed out with 18 Strength (without magic) and I'm playing with a "Legacy" or "Legends" PC with a 22 or 45 strength, how are we supposed to play the same game?
I, also, will throw in my two coppers that I have no problem with Racial maximums being different. The flavor/fluff that a dwarf can survive harsher treatment (higher than 18 Con) than the toughest human or an elf could be more dextrous or intelligent or wise than the smartest or wisest human does not bother me at all.
I am also not opposed to goign back to the B/X, BECMI idea of Racial minimums...that your "average" adventuring elf is automatically, somewhat more dextrous and/or intelligent than your "average" adventuring human. You must have minimums to play X race was another good way of incorporating flavor.
The problem there, as we've seen all over the "race" threads, is that not everyone likes or necessarily wants the races to have the same/"built-in" fluff/flavor.
The bonus modifications, in general, I would make sigNIFicantly more rare, less accessible. MAYBE a [1] single Racial Feat/Skill/Theme that allows +1 to X. MAYBE a Racial Mod but not more than +1 to X (and NO, humans don't get this. I'm a little tired of the argument, these others races get this so humans need it too for "balance". Humans are the Baseline. Bonuses/penalties work off of what humans are capable of. Say, well if an elf get +1 Dex. and a dwarf +1 Con, then a human gets a +1 they can put anywhere they want just doesn't make sense, imho.). MAYBE a magic item or two that will effect/amp up X or Y ability, without which the hero goes back to whatever they started with.
Maybe ability mod's should be built-in to class? A Fighter automatically gets +1 to their Str., Mages +1 to Int, etc...and/or ANY class may add +1 to their Con or Cha (for those extra "tough" or "charmign" folks) if they desire. Something like that for Humans...?
But PCs shouldn't be able to, from the start of play (unless you're playing epic/superhero/Legends type of game) with +4, +10, +62 to any of their abilities, no matter how many feats/skills/items/what race they are/have.
So...yeah, if I had a starting party of 1st level guys with a human fighter with 16 Str.(straight roll or placement or however you determine your ability scores), a dwarf Fighter with 19 Con. (roll of 18 with +1 for being a dwarf), a human Cleric with 17 Wis. (because she rolled 16 and took the "Contemplative" theme to give them +1) and an elf Mage with 19 Int. (rolled 17, +1 for being an elf and +1 for the "Bookworm/Researcher" theme)...not a magic item in sight, I would think this is a solid/normal party.
When they get a couple of (very rare) magic items and the Fighter now has Ogre Strength (what is that, 20?) and the Cleric has up'ed their Wisdom to 19 with a Pearl of Wisdom and reading some Tome, that makes total sense.
But they still will appear to pale in comparison of the other group with the human barbarian of 25 Strength and 28 Con, a 30 Dex halfling thief and a sinister tiefling warlock whose pact afforded them a 40 Int.
Rambling...I know. Working off of coffee #1. But I guess my point...is...what is my point?
Options good. Endless mods bad. Caps...erhm...wherever you like 'em for your game. Yeah, I guess that covers it.
--SD