I think that a Ruby Spears' Mega Man/80's Teenage Mutant Ninja Turtles crossover would be pretty interesting.
Here's what I imagine would happen, if such a crossover took place.
A priceless artifact is being held in a museum, and thinking that Dr. Wily and his robots will try to nab it, and use it for evil purposes, Mega Man decides to protect it.
The next day, trouble strikes the museum, but instead of Dr. Wily and his robots, Mega Man finds Shredder, Rocksteady, Bebop, and some Foot Soldiers as the culprits. A fight ensues, Mega Man easily destroys the Foot Soldiers with his blaster, and Shredder and company retreat. After the fight is over, April O'Neil, a reporter from Channel 6 News, has the whole fight on tape, and is about to interview Mega Man, when Dr. Wily and his robots strike the museum. Mega Man is about to stop them, but four figures come out of nowhere, and start to fight Dr. Wily's robots. Those four figures turn out to be the Teenage Mutant Ninja Turtles! The turtles have some difficulties at first, but eventually turn out victorious, sending Dr. Wily and his robots retreating. The turtles were expecting some trouble at the museum, but were expecting Shredder and company to be the culprits, instead. Mega Man tells them that he must have dealt with them earlier, and tells the turtles that they did a good job defeating Dr. Wily and his robots. After April tells Mega Man that the turtles are good friends of her's, Mega Man decides to take them back to Dr. Light's lab.
As the heroes had thought everything was over, things were about to get worse, as Shredder and Dr. Wily run into each other. Finding out that they each had a run-in with their worst enemies, the two villains decide to team up.
Back at Dr. Light's lab, Dr. Light and Roll are greeted by April and the turtles. After saying hello to each other, Donatello tells how excited he is to see how much equipment Dr. Light has, and Dr. Light tells Donatello that he's glad to have met someone else who's into science as much as he is. Dr. Light then asks if the turtles would like something to eat, and the turtles say yes. They, of course, say that they want pizza.
Back at the Technodrome, Dr. Wily is amazed at the Technodrome, and says that it's almost as good as something that he would build. To help them get the artifact, Dr. Wily decides that it's a good time to build his Samurai Bots. With Krang's permission, Dr. Wily takes a look at the Foot Soldiers' schematics, and tells Krang that he can improve them, making them almost as good as his Samurai Bots. They then decide to make an army of Foot Soldiers and Samurai Bots to help them get the artifact. Deep down, Krang feels angry at Dr. Wily for bragging that he's superior to him.
Back at Dr. Light's lab, April and the turtles are about to leave, when, on the tv, a report comes on saying that robots that look like ninjas and samurais are attacking the museum where the artifact is being held. Mega Man suspects that Dr. Wily and Shredder must have joined forces, after thinking that the Samurai Bots look like creations of Dr. Wily. Mega Man and the turtles, along with Roll and April, go to the museum to stop the robots.
When they reach the museum, Mega Man, Roll, and the turtles fight the robots. When Leonardo tries to slice a Foot Soldier in half, his sword has a hard time cutting the robot. Leonardo tells the others that the Foot Soldiers' armor is stronger than before, not to mention having improved fighting skills. Dr. Wily, who, along with Krang, is watching the battle, tells them that he has improved the Foot Soldiers' armor and artificial intelligence, making them stronger than before, though they're still not as strong as his Samurai Bots.
Donatello, with some help from Mega Man, has defeated a Samurai Bot. Donatello decides to bring the fallen Samurai Bot back to Dr. Light's lab so that they can find a way to defeat the robots. While Donatello flies back to Dr. Light's lab on Rush, the others continue fighting the robots. The large group of robots tire the heroes out, and a Samurai Bot gives the artifact to Dr. Wily. Dr. Wily says that now, with the artifact, he can make a super weapon to help him take over the world. Krang then grabs the artifact out of Dr. Wily's hand, and says that he'll be the one who rules the world, not him. While the two villains fight over the artifact, Donatello returns with a remote control. Donatello tells the others that while he and Dr. Light were examining the fallen Samurai Bot, they found a circuit inside of it that can overheat and explode. Donatello then states that this remote control should cause the Samurai Bots, and the improved Foot Soldiers, to overheat their circuits and explode. Donatello presses the button on the remote control, and the robots overheat and explode.
Michellangelo then uses his grappling hook, and takes the artifact from Krang and Dr. Wily's hands. Dr. Wily then tells Proto Man, Guts Man, and Cut Man to retreat, and they hop into Dr. Wily's ship, flying back to Dr. Wily's castle. Krang and Shredder also decide to retreat and head back to the Technodrome. Bebop and Rocksteady, while trying to catch up with Krang and Shredder, complain that they'll never see their new friends, Guts Man and Cut Man, ever again.
Back at Dr. Light's Lab, Dr. Light, with permission from the museum, decides to destroy the artifact so that it won't fall into the hands of evil ever again. He also states that Dr. Wily probably won't ever team up with Krang and Shredder ever again, since they don't get along so well, which brings a sigh of relief to Mega Man and the turtles. April then says that she taped their battle, claiming it to be the story of the century! Dr. Light then thanks the turtles for their help, and gives each of them a pizza as a token of his appreciation. Michellangelo opens his box of pizza, and is about to get a slice, when Rush jumps him and takes his pizza. As Michellangelo chases after Rush to give him his pizza back, everyone else just laughs at the event.
I probably had much more details than you wanted, Master Toon, but I had so much fun typing this that I had to type the rest of it out. So, sorry about that. :sweat: