After looking up on the Minecraft EULA Restrictions i am thinking of another way you guys can make money to support the server. You guys can make it so pets are able help you in battle. I saw this in a local server and i thought it was really fun. It basically makes it so your pets have levels. You get to chose whether the pet is a ranged attacker, a melee fighter or a rideable mule. The idea is that every time you kill a certain amount of mobs or interact for a certain amount of time, your pet increases in health, damage and new abilities. I think this can help, But to Bypass the EULA you would need to give out a free version of this pet so that donators have no physical advantage and only gain from cosmetic services. So let's say you give out a (ugly :I) free pet that has the same abilities as the pets you sell at the donation shop. But the pets in the shop look cooler (let's say they have a range of effects on a small wither pet) that costs money. Hope u guys like it :I pretty sure the plugin is unique though so slight chance this would be happening.
I would have to say -1 for this reason, Battle pets, I am very concerned about exploiting/lagging the server. Sat you have 50 people battling at once it may just lag the server which can be very annoying. And we already have the pet feature so, no.