I quit reading about a third of the way into the second page.

For me, "mainstream" equals "human figure with the same options and potential as any other."  With all the morphs and such most any figure can accept, why buy more than V4 if she can be "mutated" into all of the versions currently available for sale?

Convenience.

The more convenient any product is, the more likely it will appeal to hobbyists generally, and the professionals would possibly also like the ease of using something they do not have to "fix" before they can "do something" with it.

If, for example, you were to make a new figure from scratch that can wear all of V4's clothes, that would be convenient for those who have a "closet" full of V4 stuff but want something really new and not just another (good) version of V4.  It would not need to be supported as a stand alone.

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From what I did read, you are looking for something either to make (the figure) that others will help you to support, or you are looking for something already made that you can support well enough not to be wasting your time as a business.  Either way, I have no idea what specifically would work for you, but I would suggest considering something non-mainstream but still viable, such as accurate animals and their equipment.  Horses and riding gear, battle critters and their armor, fish and their tanks with treasure chests, etc.

This would risk your efforts being a fad but, unless something were requested by a lot of people before it was made, everything starts out as a fad - the "mainstream" are the ones that last.