A - Find a new planet with an advanced alien civilization on it. But when you go back to Earth It's been taken over by Cthulhu.
B - Have a bunch of alien advanced alien weapons, but Earth is now infinitely being attacked by mythical creatures. Wendigos, ghouls, Phoenix, Goblins, ogres, and the list goes on.
C - You can travel between dimensions, but each time you do it kills 10,000 kids in each dimension.
D - We can prevent natural disasters, but now Earth has a better chance of being destroyed by massive meteors.
E - You created a successful time travel machine, but each time you back to the past or forward into the future and return to the present something massively different will occur/change.
F - You actually find god or gods, but the consequence for that is everyone on Earth will die and go to hell for eternity. You'll be fine though, in heaven per say.
G - You find a cure that can heal literally anything; everything. But now the population is so high that resources are running out at a massive rate.
Hope you enjoy the WYR. New WYR [u]every[b] Wednesday[/b][/u]. Peace.