I got this idea from SC2 community.

What if IMMs announce some nice and neat event or quest which would happen if players donate, let's say, 50 $ (just for example).

Players are more motivated to donate. IMMs are more motivated to run such events because, you know, money is nice.

I am actually working on something that won't cost you a dime.

I'll be honest too. At least four times in the last two weeks Ive seen team avatar come down and do some really cool immteractions for players, even if its just a quick rp session. Once was in the form on a mob being taken over, the other three were just really cool little sessions, weather for my character or another. I've noticed the imms trying to get their foot down on the mortal plane a little more, and do interacting. Just little things like these randomly all over the mud will get the playerbase amped up about their chars. I know it's kept me interest in mine and helped me define my role better.