ETA:  jolrei, if you don't like a Sim's wish, just right-click it and it goes away.  It (probably) won't come back unless it's associated with a trait or lifetime wish of the Sim.  Also, one of my Sims had been staying home a lot after retirement, and he got a wish to get out of the house because he was feeling stir-crazy ;-)  I sent him to the library.  He was a bookworm and loved hanging out there.

Didn't they say the death flower let you beg for your life when you die?  You just keep it in the inventory until it happens.
To make playable ghosts I think it is that Oh My Ghost! opportunity from the science center.
I did not try to verify either though.