Omnistone is supposed to influence the playstyle of the player, what you're proposing is in the complete opposite direction of what riot intended for this rune.
Fixed stats are dull and unimaginative, and they don't add anything new to the mix. You will be overpowered early because you'll have way more stats then other laners, and fall off later because your rune doesn't scale. Its poor design.
Maybe if the was something like:
Omnistone provides a stack on fixed intervals (reduced by takedowns/assists) towards a random stat. If a stack would cause you to exceed a stat limit the stack will be moved towards the stat with the least amount of stacks. Additionally stats that are beneficial to your immediate situation will have a higher probability of being boosted.
If you are consistently taking heavy magic damage: MR +
If you are consistently taking heavy physical damage: AMR +
If you are nuked too quickly: HP +
If you are a tank: HP, MR, AMR +
If you are winning lane: AdpF +
If you are an ADC: AdpF, AS +
If you are a mage: AdpF, CDR +
If you are a jungler: AS +
So on so forth.
Gain a Stack: Every 90 seconds
Takedowns: Reduce remaining timer by 10 seconds.
Assists & Epic Monsters: Reduce remaining timer by 5 seconds.
Large Monsters: Reduce remaining timer by 2 seconds.
Base:
Health: (Stacks) x 15
Adaptive Force: (Stacks) x 5
Attack Speed: (Stacks) x 10%
Armor: (Stacks) x 10
Magic Resist: (Stacks) x 10
CDR: (Stacks) x 5%
In a 20 minute game you'd have a 20~30 stacks if you played relatively well.
Assuming it was a balanced distribution:
Stacks = 3~5
Health: (Stacks) x 15 = 45 ~ 75
Adaptive Force: (Stacks) x 5 = 15 ~ 25
Attack Speed: (Stacks) x 10% = 30 ~ 50%
Armor: (Stacks) x 10 = 30 ~ 50
Magic Resist: (Stacks) x 10 = 30 ~ 50
CDR: (Stacks) x 5% = 15 ~ 25%