Sounds like an excellent plan - I could well be interested in one come the day.
I would guess for a portable device one would want to keep this to a minimum.
Either just a stereo 3.5mm jack (not my preferred option)
VGA/DVI/HDMI port - whatever seems best
One or at most two (mini)USB port(s) - as long as the USB had enough bandwidth a hub could take care of Joysticks/Mouse/Keyboard
a stereo 3.5mm jack
Either it's for personal use only or it can be used with friends in which case it would likely need all that potential functionality.
Personally I'd go for a MiniSD or MicroSD card as that's going to be more than enough for a portable Amiga and would be useful for transferring software. But if you're thinking of potential upgrades then a 1.8" HDD makes some sense.
(I guess that depends on how much you envisage the firmware developing)
Personally (being a grumpy old sod) I despise all portable devices that come with internal speakers - it's bad enough having to sit next to someone on the train listening to "chi, chi, chi" because their earphones are up too high*...
...also less hardware to pack in the case.
Looking forward to developments...
*Revenge comes to all these people in the end - an old age when they can't hear anything at all.