Great minds clearly think alike Gilamar Skirata since I had down today to do some rebellion updates which take elements from what you suggested.
As part of our updates on all things map and faction related, some small changes to rebellion rules.
Firstly, the minimum period for a rebellion has been upped to 14 days to bring it in line with invasions. This may be extended if both sides agree as with invasions.
Next, the following only applies to successful rebellions against Recalled Major Factions. It does not apply to other types of Rebellions (from Dominions etc).
If a Minor Faction successfully wins a Rebellion against a Recalled Major Faction and chooses to go Major they may start with up to 3 contiguous hexes total from that Recalled Major Faction. This may not include the recalled faction's capital.
Recalled Major Factions are quite rare, rebellions against them rarer, and victorious rebellions against RMFs has only happened once I think. Therefore, let's reward those rare cases with a couple of extra hexes.
This will take effect on the 1st of August and will not affect Rebellions declared beforehand.