The hardest problem is not the board definition, though... It is to find a (Chinese) company ready to make these boards at these low prices
The only possibility would be to get in touch with one of the original RP/BP manufacturers and propose a list of enhancements that could either increase their sales and/or margins significantly because of differentiating features and/or lower BOM costs.
Someone living in Shenzhen in this forum ready to accept this mission? He/she would receive the highest consideration from the community!