Spouse's response: sounds like you have some good ideas, but it sounds like a "crazy people" church. Many churches that have that structure have gone off the deep end theologically. So you really don't want just anybody getting up there to preach, because they could take some pretty horrible positions and use the Bible to justify them. Many churches get around this by requiring someone who preaches to be somehow ordained.
Good point, Spouse.
So, how to get around that? I do want anyone to be able to get up and preach, as long as they are Biblically grounded. I don't want them to have to have a seminary degree, though they should be knowledgeable of the Bible and basic Christian beliefs. I also don't want people to get up there and push an agenda, using the Bible to support it - the idea is preaching on the Bible itself.
You could have a requirement of the number of times they have to attend before they can preach, and have some sort of governing body approve them. This would probably involve knowing the topic ahead of time and getting to know the person. However, this puts more power in the hands of a governing body, rather than little small groups within the larger group being equal, with each small group able to do what it wants. (so that gets around the idea of "you'd need a governing body anyway to make the church's decisions".). You could do it as a small group model, where the group leader must approve of the person, though that definitely puts the power in the hands of one person, and if your group leader didn't like you, you'd be unable to preach.
So I guess a small governing body of people chosen by the church would be the best option. Although then people would choose their friends instead of the best leaders. The governing body would be small enough to be doctrinally consistent (even if they come from different denominations, as long as there are no major differences in theology) but large enough to not hold to just one theology and consider all else wrong, and large enough to incorporate a diversity of opinions to balance each other out. Grr, having to set up a government here, which is exactly what the Founding Fathers went through when they set up the United States - how to set up a government which fairly represents the people and their wishes but doesn't ramrod over a minority? In this case, it's how to set up a board which represents a diversity of opinions but not so diverse that you become inconsistent with Scripture and thus stray theologically.
Perhaps publishing by-laws, or defining theological lines? Of course, those theological lines would be broadly drawn (e.g. no official stance on whether baptism is necessary for salvation, or infant baptism, or transubstantiation, or any other non-salvation-oriented denominational issue), but would be clear on certain basic Biblical beliefs, the set that someone MUST believe in order to be considered a Christian.