Herald of tzeentch chariots are made from discs of Tzeentch pulled by screamers and the fantasy Lord of Tzeentch works well as a herald with an arm swap from a horror sticking out of his robes.
A good herald of Khorne is the old metal bloodletters. They are significantly chunkier than the new plastics but maintain the Khorne feel. A fantasy chariot pulled by a single bloodcrusher or 2 fleshhounds would work well.
Chariots of Slaneesh can be pulled by seekers if you can find the models but new plastics have apparently been sighted and will hopefully be released in 2010. If you cannot wait try gamezone miniatures who make some similar mounts for their daemon cavalry.
Virtually any warhammer fantasy chariot will work, or you can even go simpler and just make some sort of enclosed platform on wheels being dragged by either a few daemons or a pair of warhammer fantasy steeds.
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