So chaff troops are usually the first casualties once the game progresses from early to midgame since they have the rather fatal flaw that they replenish just as fast as elite troops, but die easier and dish less damage and so become simply cost-inefficient. So here's a suggestion of how to increase their value:
Let them replenish to full automatically after every battle you win!
Chaff troops, that is Goblins, Skavenslaves and Peasants are mainly meant to soak up damage and screen for more high value troops. Using them in this way however is, as mentioned above, not a very efficient thing to do as they will get shreddered and then you have to wait for a while for them to either replenish or buy new ones, as understrength chaff troops are very ineffective on the battlefield, which means whatever momentum you gained from your victory is immediately lost. You should not lose momentum because your meatshields were used as, well, meatshields.
That's why I think something as I suggested would help with this issue and make chaff troops useful beyond the early game. Of course, this should also be combined with severely toning down the replenishment of veteran and elite tier troops as well.
Excluding the undead here because they can already replace their meatshields immediately after every battle with the Raise Dead mechanic (and TK can just spam no cost slave stacks).