People have already suggested this, but after seeing Jonh’s Force ghost tests so far, I definitely think it would be cool to see the ghosts help Rey further. What I’m thinking, is after Rey starts deflecting with both sabers, we start seeing a Force ghost aura around her. And the first time she pushes forward, we get a very quick glimpse of Luke’s face over hers. The second push, we briefly see Leia’s face. The third push, Anakin’s (it could even be quick flashes of multiple Jedi). And in the shot of Rey pushing the final blast of lightning back at Palpatine, we could see that blue aura around Rey fly away from her, through the lightsabers and lightning, and into Palpatine. This is when Palpatine explodes. Basically, the Force ghosts of the Jedi use their energy to purge the spirit of the last Sith, once and for all.

This way, Palpatine doesn’t die just because Rey deflected lightning in his face, but because of something more metaphysical, that feels like a culmination to this battle between the Jedi and the Sith. Palpatine has been a vessel for all the Sith, and for this brief moment, Rey becomes an avatar for all the Jedi.