I like the idea, and I’ve seen similar proposed before. My thoughts below.
Non-combat move
A new phase, between Conduct Combat and Mobilize New Units, called Non-Combat Movement. Any land unit owned by the power whose turn it is that: 1) did not move in the Movement phase (renamed the Combat Movement phase), 2) did not fight in the Conduct Combat phase, 3) is in a friendly territory, and 4) can trace a path of friendly land territories between its current territory and the capital territory of the owning power, may conduct Non-Combat Movement. Assuming the four conditions can be met by at least one land unit, the owning power may move to its capital territory any or all land units meeting those prerequisites.
It could be worded better, but the gist is: if a land unit belonging to the power taking the turn did not move, did not fight, is not on hostile territory, and has a valid path home, then after Combat and before Mobilization that unit may be picked up and placed back in the capital territory for that power.
Something like that.
-Midnight_Reaper