Might work if the "defend" thing was limited to original space , not conquered territory.
We can use the Cartel maps to mark original space.
This could get really complicated in hexes that are being fought over and flipping back and forth. Lose a combat get banned from 7 hexes, lose the save combat 30 seconds later in the same hex just be banned from 1. Doesn't make sense.
What I would do is apply this to planet and base hexes only. This gives the defending race an advantage protecting their home planets and defensive installations, but gives now special bonuse for open and empty space where there should be none.
If people were wanting a bit more I'd suggest that being kicked out of any hex surrounding a planet that was an origional enemy world would result in a ban for the planet hex and all other hexes surrounding that planet. For bases I wouldn't do that since they can be placed by players.
I also like this as it would make certain planets close to others even harder to take, think Romulas and Remus, or the 3 homeworlds of the Gorn. It is realistic to think of these as particularly tough targets with lots of concentrated defensive units and aid cloe by.