"I'd like to design a new development board with a 8051 microcontroller's familly for general purpose."

I think there may be some confusion as to what exactly you mean by "development board" here.

When we talk of "development boards" here, I think most of us envisage a general-purpose product sold to assist in development, or evaluation of a particular chip.

The point of such a "development board" is that it allows you to start with some known good, working, documented, debugged, supported hardware; this gives you a point of reference when your software doesn't work on your own first hardware prototype - you can try it on the dev board, and hopefully determine whether you have a hardware or software problem.

Are you making such a "development board," as a product for sale?
Or are you seeking to make a common hardware platform for use by you within various of your own products?

Note that many processor vendors publish "Reference Designs" - which consist of a full schematic etc; might be your best starting point?