IT's hard to define for the A1200.
Yes, the chance for having USB can have high priority. Equally with a well supported soundcard. Also netaccess is a keypoint (honestly don't know anything about wireless networking, so a good tutorial maybe, although it doesn't really a hw to be built)
And still, withount any CPU and/or RAM expansion the A1200 is a giant with one arm.
I also think that the most important thing recently for any A1200 owners to not timber the case into pieces in their shed, more preferred anything which is at least looks like a factory default upgrade.
In short: I don't know