Assuming you do not want to do work inside the panel, on your own, or
that doing so is a code violation - I don't know if this is up to code
but while the electrician is in there I'd have him add enough circuits
to handle the future expansion. Either run one big circuit to a branch
box at the end of the house where the new addition will be, or run some
circuits to junction boxes whereever is convenient. Then you can tap
into the junction box without having to get into the new panel.

Basically have the pro do all the work inside the new panel but run
circuit(s) which would be left 'off' for future growth.