Make it possible to gather support after 10000
Add another milestone (15000? 20000?). If an idea gets to this milestone while in review, there will be a guarantee that it will be a set. Or that there will be a set based on the same thing. Example: Boeing 747 (AN EXAMPLE) gets 10000 supporters and goes into review. However, it's very popular and it reaches another milestone. Then, LEGO will make a Boeing 747.
Unfortunately, we will never be able to guarantee a product idea is made on the basis of a fan vote. Especially not when it’s submitted based on an intellectual property to which rights would need to be obtained first (if it’s even possible).