Hmm interesting thoughts, considering that their income they receive with votes I think some reward would be an excellent the version of bestowal or an act of courtesy. Also for steemians to vote for their witnesses would be more attractive and then they would spend all their 30 votes, and not keep them to themselves. But I think all of us wonder to know what witnesses think about it?...
Also about witnesses rewards you can read here https://steemit.com/utopian-io/@semasping/accusta-witness-s-rewards-v-0-7