I'm really, really sorry if this has been suggested already. I looked through a few pages and I didn't see it or anything that seemed like it so I thought I'd just throw this idea out there.

What's the point?

The point here is that in the wild, young males will try to take over a pride to become king. I believe having the sub try to take over is the closest we can get to that, as usually the other lion is not from the pride

People may not want to wait to retire their king and do not have the GB to retire him. They may want to open up a space for a new sub, but don't really want to get rid of their subs nor do they have the GB to get a new space.

How does it work?

Well, there will be a button on your lioden page or on the sub's page. This will not happen automatically, so it is 100% up to you.
While I'm not entirely sure how the fight itself will work, I imagine it will be similar to the pvp fights for territory/experience/impression.

If the sub male loses he will be chased/killed and will be unable to try again, so use this feature wisely if you don't want to lose your sub.

The chance of the sub winning is up to the staff, however it will most likely be 50% or lower.

There will also be a cool down to prevent people from abusing the feature; after trying, there will be long cool down period(possibly three in game years or higher) before another can try again.

Additionally, subs must be three years old in order to try to dethrone the king and cannot try to take over if he is on a patrol.

Only one sub can try at a time as there can only be one king.

Finally, cubs will not be killed by the new king unless you do it yourself manually. There could be an option for it if an evil sub manages to dethrone the king, but again, it will be optional.

Feel free to send in some suggestions of your own if you feel I missed something!