So we have three Guardians (breeding, food and rune Guardians), but what about adding a fourth one, which increases your gold production (25, 50, 75 and 100%), but, instead of gold, it costs food to activate (let's say 3 million food every time to activate it for 2 days).

Any thoughts?

Rune guardian can affect food production already.

@SkeelMaster did you mean gold production? Well if you have gold runes on most of your monsters, then yes (but I'm not going to equip high lvl gold runes on my monsters, instead I convert those runes to speed or strength runes), it can affect gold production. But this is different, because it affects your whole production, not just the monsters with gold runes.
Let's say you can collect 40 million gold in 12 hours. With this you could collect 80 million (at max rank).

I think this is a good idea, using runes guardian to use for a gold farm is less than effective for it, providing a slight, maybe even negligible bonus to gold generation.