A development board should be forgiving. Thus I recommend to add protection against most probable errors like wrong polarity of VCC as well as overcurrent protection to the GPIO (PTCs, limiting to 20mA). I'd also not use a crystal for development. Internal oscillators of IC are sufficient for development. They'd be only necessary when using baud rate crystals for high speed serial communication or if you really need the 20MHz.
If the latter were true I suggest to use a more recent ATMega chip with additional periphal features like built in DA converters or improved AD rates.
Maybe you outline more clearly what »development« means in the context of your circuit.
There are features like oscillator calibration and individual capacitance of traces to XTAL that maybe require other Cs than 22pF. I'd also put in a spare/ socketed level translator or hex-buffer for I/O lines connected to other devices – saves the µC by sacrificing a ten-cent-part.
And AREF shouldn't travel all around the final layout or be mixed with all other signals. Instead you want a short trace with a ferrite bead/ inductance and good decoupling also from digital ground. The cheapest thing I often do here is a reference in a TO-92 case, e.g. LM4040 or LM385. And I prepare a few voltage dividers to be populated as needed and solder joins for AD0-3 inputs.