Imagine! (read only if you finished death note)
What if, and hear me out, what if...
L knew about the rule that if your name is put into the death note wrong three times, you can't be killed by it anymore.
And what if he ripped out a page secretly and wrote his name wrong three times, or has Watari do it if he can't himself.
Then he couldn't have been killed by the Shinigami.
And what if he still faked his own death and disappeared, everything in the series continues as we know. Only for L to appear again in the showdown where Light is killed. Like, "Hey there old friend".
And then you get an OVA about what L did all this time, watching the story unfold from the shadows, dropping hints here and there.
And what if those hints are hidden as Easter eggs in the scenes of the actual series, but you only catch them in a re-watch because then you know.
Just saying, that could have been the ending.