The only possibility that Lan Di would die by Ryo hands is that he dies from falling over a Clift or that he falls on water and maybe Lan Di does not know how to swin but i really doubt that...
The only person Ryo sort of killed was shenmue main enemy Chai because he felt on the water which happened again during the boat trip... Dunno if he died.
For Ryo revenge is not killing Lan Di in his mind revenge is fighting Lan Di and defeat him in battle and sort of humiliating him.
Lan Di on the other hand is a high ranking Chi Yo Men and he is know to be ruthless. He has killed people before he killed Ryo father.
I trully believe shenmue series would not end on a third game or that we would fight Lan Di....
Shenmue III is to meet and defeat Niao Sun.
Shenmue IV is to defeat Hong Zi Ming, Xi Yiu Brother.
Shenmue V is to fight Lan Di.
Shenmue VI is to defeat the leader of the Chi Yo Men the main villain.
and that one of the reason we not getting a third game..
We would actually fight Lan Di till Shenmue V. Sadly I doubt Shenmue would reach that far
with the current situation of the third game.