Ringo Starr is confirmed to be capable of using the elfstones, and has them in his posession.
As a preeminent Beatles expert, let me tell you what this means:
he might use them to wreak vengeance upon the other ex-beatles
he might turn their own power around and destroy the elfstones, thus eroding one of the pillars of elfreality and causing an elfcollapse.
if he can access the FULL power of the elfstones, he could possibly bring other beatles back from the dead, torment them, kill them all, and resurrect them all all over again, in an infinite loop.