Here's an idea:
The veggie feeding bar now specifies the light intensity in %.
A new bar allows to specify how much free waste there is in the sim.
Pond mode puts more free waste in the lower side, opposite to the light intensity.
(nonzero) .getwaste -> gets the free waste as factor of body size.
Waste digestion turns a % of the total waste into energy each cycle: the % is specified by the light intensity. At 100% light intensity, a bot converts all the waste it gets into energy. Waste digestion works for all bots, since even predators can use small amounts of minerals.
At 50% intensity, and 1 point of waste per 10 body points, the A bot gets:
A: body 200
Turn 1: waste: 20, converted waste: 10
Turn 2: waste: 30, converted waste: 15
Turn 3: waste: 35, converted waste: 17.5
Turn 4: waste: 37.5, converted waste: 18.75
That means that, if the veggie allows waste to build up, it will maximize the energy. If it grows, it will get more waste and energy too, though maximizing the energy has an increased risk of bad waste effects.
Bots will choose then between sensible DNA that doesn't allow waste disruption but allows complex behaviour, and dumb DNA that can manage higher amounts of waste but it's too random to compete against the sleek hunters.