For the campaign rewards or levels an idea you might consider for those categories is when you select them the unit/character goes up in cost according to the table.
So for example:
Melee Table
1.
WS +1 +15pts
2. A +1 +15pts
3. S +1 +15pts
Ranged Table
1.
BS +1 +15pts
2. W +1 +15pts
3. I +1 +10pts
Support Table
1. T +1 +15pts
2.
Ld +1 +10pts
3.
Sv +1 +15pts
Basically, you roll a 1d6 and 1-2 = 1, 3-4 = 2, 5-6 = 3.
However, when you roll and get your choice it adds that many points to his cost.
BUT...if you were to roll the same stat twice, the price doubles.
Example:
Wolf Lord hits
lvl 2. You pick the melee table and roll the dice.
Your dice comes up 3. Meaning you get +1 Attack to your standard stats at +15 points to the model.
Now, Wolf Lord hits
lvl 3. Again you pick melee and roll your dice and get a 4. Getting another +1 Attack to your stats. Only this time instead of getting it at +15 points, it's now +30 points for the second +1 Attack.
With this, to limit the character getting to powerful, you can either keep doubling any subsequent repeat choices OR you can cap it off. Saying you can only get +1 Attack 2 times before that option becomes unavailable to him.
Hope this is useful. Good luck on your campaign.
Cheers~