Such is life, work around it. Your son deserves that throne.
Well, if I can't choose my heir, I might as well kill of all other heirs until the one I want is the oldest one around.
Souvereign: 'Hey son, I made this deal with the old, dangerous dragon living in that swamp over there. Could you go and have a talk with him?'
Unfit heir: 'Sure dad, no problem...'
Souvereign to dragon: 'As agreed, I'm sending my heir over to you as a sign of goodwill, do with him as you please.'
Unfit heir: 'Hello dragaaarghhhhhh....'
Personally, I like the 'Sovereign dies = game over' option. Knowing you have won the game and having to search for every last heir around could get very tedious very fast.
I don't understand why. We gonna have a dynasty system afterall. It's pretty logical if the sovereign dies, his eldest child/some other child chosen by the player itself will be the next sovereign/ruler.
It might be logical for an heir to become the next ruler, but why would they be able to channel? As the story is planned for now (I think) the ability to channel is very rare, so the children of the channeler might / will not be able to channel at all. The sovereign is able to imbue someone else with his essence, but that doesn't make that person a real channeler. So, if the heir is a lot weaker compared to the original sovereign, and another player was able to kill that sovereign, why would you like to play on with a new ruler that's so much weaker? You might a well stop playing.
In the end, I wouldn't mind if this would be an option and maybe we'll be able to beta test some if this and see / feel how it plays.