Deserting/rebelling unit: improvement idea

I apologise if this has been posted before, I did a search and couldn't find anything.

I believe the current system whereby a rebellion occurs and a garrison switches sides and is immediately usable by the other side, at full strength, is unrealistic.

If the garrison is weak and a rebellion occurs, the garrison unit(s) should:

1) remain in control of the city/province albeit with lower health/hitpoints;

2) be expelled from the province, starting a withdrawal to the closest friendly province;

or

3) desert to the enemy, but with penalties in terms of combat power, and a time penalty before any offensive action is again possible for that unit.