sbenno,
thank you for a good question. Sure, I will not be speaking about "bread and butter" VSTi like replics of Great Old Evergreen Instruments. I will not be showing to you VST reviews like http://www.kvr-vst.com does. "Marketing advice" is very simple: take first 10 most popular plugins (popular according to any statistics).

I'll better point you to the trend: it is morphing that rules in today's music industry. Morphing is most technically difficult and musically fresh concept that earn more and more attention in our postmodern world.

For me personally it will be interesting to try at Mediastation:

White Noise Additive Sinth (http://news.harmony-central.com/Newp/2004/White-Noise-Additive-Synth.html)

Cameleon 5000 Additive Morphing Resynthesiser (http://www.camelaudio.com/productsCA5000.htm)

Symptohm:Melohman (http://www.ohmforce.com/ViewProduct.do?p=Symptohm)

I tell you that you should pay attention for performance-oriented (not sequvencer-oriented for home computer work) VSTis. Mediastation performance-oriented too with all it's sliders and buttons.

And then we can speak about morphing of styles -- but it is shure not a VST story.