A funny thing I noticed a few months ago is that the majority of modern anime series has theme songs that are roughly 90 seconds in length. So, one day, I was watching an anime with a theme song I didn't particularly care for, so I decided to mute the opening theme and play a different theme song while watching the opening. Surprisingly, the visuals matched up almost perfectly!
Ever since then, I've tried many different combinations with different themes overlaid on different opening sequences. Some were decent, some didn't match up well at all; however, others were either too funny or too fitting to keep to myself.
This first video is the first opening sequence to Keroro Gunso/Sgt.Frog with the opening theme, "Guren no Yumiya," from Attack on Titan. I may upload more eventually, but I encourage you to try mixing up a few for yourself. You may come up with something I never would have.Invasion of the Demon Frogs