I got into a debate over abilities in 4e and my continued opposition to the drive of 4e to consolidate in one (or a few) abilities. The output of the debate was to try a carrot approach instead of a stick and see where that gets us.
The following would probably find its home under the "1. Ability Scores" section of Leveling Up in the PHB.
There are two types of ability increases: heroic and paragon. A heroic ability increase can increase an ability score to a maximum of 18 and a paragon ability score can increase an ability score to any value. Two heroic ability increases can be combined to gain a single paragon increase and a single paragon increase can be traded for two heroic.
At 4th and 8th level, gain two heroic ability increases.
At 14th and 18th, gain a heroic and a paragon ability increase.
At 24th and 28th, gain two paragon ability increases.
At 11th and 21st level, increase all ability scores by 1 regardless of score.
Depending on your game or your goals, you could adjust the limit of the heroic ability increase to taste. The impact would be that a player who wanted to focus on a single ability would be able to without interruption but would be rewarded if they were willing to split paragon increases up and focus on lower abilities. This would make the character more versatile overall (one of my goals) without mandating or foreclosing the "all in one stat" approach.