Hi

For my opinion, A general development board should contain
1) 8051 (ofcourse) and all necessary componments (Xtal ...)
2) 64KB Rom (Maybe one day you'll need more memory)
3) 64Kb Ram (See #2)
4) 8255 (For extra ports & Communication)
5) 8 Leds (For monitoring and general outputs)
6) LCD (To describe diferent situations)
7) Small Power Supply, So you'll be able to use anywhere
and operate other small circuits with it.

and anything you think that will be usable.
Goodluck
OK