I have one suggetion for expandabilty / using other derivatives having extar features.

(1) Instead of putting a fixed CPU socket on board instead a suitable CPU slot (Expansion connector similar to that in PC ISA , PCI type).

(2) In this connecotr we can put CPU module which contains CPU of our choice . 89s8252 module can be supplied as standard one along with 2-3 other modules for other deriva(like phillips, Silab deriva having extar features then 89s8252 etc). If somebody wants to use other deriva then this he can very well make module for that deriva.

(3) CPU module can have contain ISP progra hardware & connector. In this any method of ISP can be used like through parallel or serial port.

(4) CPU slot will have 40 8051 standards pins mapped plus additional pins for derivatives offering more port and pins. These extra can be like ADC input , PWM input/output etc. These exrta pins can be terminated on suitable expansion area of board where user can add additional hardware for using additional pins/features offered by other derivative.

I am suggesting this because once new comer experimented with basic features of 8051 through 89s8252 he will like to go for advance features supported by new derivatives