Burns is going to outlive everyone.
Besides he can't die until him and Smithers become official

Hmm, possibly. I wouldn't want him to die while the show is still running, but for a final episode I'd be okay with it.

Although I'd probably want it to be done in a not-too-serious way. For example, something really dangerous could happen to him (like getting a deadly disease or be involved in a lethal accident), but then he overcomes it. Only to then be killed by something else immediately after (like a heavy object falling on him). Bit of black comedy.

That said, what they could do is leave his fate open-ended. Make it look like he dies but give a subtle hint that he may still be alive. Just to mess with viewers.

would be nice to kill a important character because they have so many season already just kill him already