This question is actually a lot harder than it seems.
More incentive for (older) players to come help as pest control seems nice, but how on earth does one balance it? The cash reward is (partly) here for that reason, and clearly does not work. A lot more gold however, would make it very unballanced...
That leaves us with 2 options; ironmans supplies or untradables (cosmetics).
-ironman supplies seems good but is also impossible to balance. In no world should pest control be the best way of gathering any sort of materials, as that would defeat the purpose of ironman mode. So then we would end up with a half-decent way of getting resources nobody uses cuz there are more efficient ways in the game.
-cosmetics would work, for a little bit. Theres not many people actually aiming at cosmetics and unless the cosmetics would cost A LOT of points (or for instance a pet would be extremely rare), nobody would want them as they are not exclusive. Then, those very exclusive items would only be grinded out by a very small portion of very dedicated grinders, not helping pest control activity too much.
The xp idea i do like, but yet again is impossible to balance. Pest control should never be the fastest xp in any skill, resulting in very little players actually using the method purely for xp.
Concluding the above id say the 2 best options are; a pet chance (as long as it looks sick people will grind it) or a combination of xp and untradables. (Which would result in a decent way of training whilst getting some cosmetics)
Worth all this hasstle? Im not sure..