Batman x adult Ben 10
Adult Ben Tennyson in his late 20s - early 30s.
(No ultimate Aliens, No fusing aliens, Can't turn into DC aliens, Ben 10 Aliens don't exist in the DC universe)
List of his aliens (Heatblast, Stinkfly, XLR8, Ripjaws, Upgrade, Graymatter, Fourarms, Wildmutt, Ghostfreak, Diamondhead, Cannonbolt, Wildvine, Spitter, Buzzshock, Arctiguana, Blitzwolf, Snare-oh, Ditto, Upchuck, Waybig, eye guy, Frankenstein, Chromestone, Jetray, BigChill, Goop, Spider-monkey, Humungousaur, Echo Echo, Brainstorm, Swampfire, Alien X, Rath, Lodestar, Ampfibian, NRG, Waterhazzard, Terraspin, Armodrillo, Chamalien, Juryrigg, Clockwork, Eatle, Shocksquatch, Feedback, Gravattack, Bloxx, Crashhopper, Ball Weevil, Walkatrout, Peskydust, Mole-Stache, The Worst, Kickin Hawk, Toepick, Astrodactly, Bullfrag, Atomix, Gutrot, Whampire ) (They all wear the OG back and white Outfit)
After all of his family and friends were killed in a war he died trying to save someone.
He wakes up in an unknown place where he can see he was being experimented on, He transforms into XLR8 and goes through everything in the laboratory finding out they experiment on dead kids, kidnaped kids with powers and they found him dead and tried to access his omnitrix but no matter what they did they weren't able to access it.
He then turns into Heatblast and destroys everything killing all the people inside aswell. (this Ben has no problem with killing)
He then transformed back into XLR8 and runs all over the strange city going so fast He can't be seen even on cameras.
Not recognizing the city he finds a library breaks in and using XLR8 goes through all the books trying to find anything that talks about his home.
But after going through every book and nothing is the same he comes to the conclusion that he is in a alternate universe.
He finds out he is in a place called Gotham city, That there are heros, Aliens (none of his) People with power are called metas, Heros here don't kill, about the vilians especially the Gotham's ones etc.
Normally he would have tried to go back to his own universe but everyone he loves was dead so there was no point.
He finds a run down place to live and in a few weeks has accidently became a vigilante after transforming and helping when he was anyone in trouble.
And since Ben transformed into different aliens to suit the job, people think its a group of vigilantes as they notice the omnitrix symbol on all the different vigilantes. ( the OG back/grey and white symbol)
The people of Gotham like the group of vigilantes especially Crime alley as he is always helping even during the day.
They have favourites out of the group of vigilantes, Four arms, Humunagsaur and Wildmutt are the favourite of the kids as they play with them.
Heatblast is the favorite of the homeless as he flies around making fires that are extremely warm and don't go out of control for them.
The working girls favourite is Kickin Hawk and Rather as they teach self defense. (They loves how Rath talks)
And more.
When Batman finds out he thinks it's a group of vigilantes aswell ans he tries to track them but he wasn't able to.
He meets red hood a couple of times and even saves his life. They become friends and after an incident Jason reveals his identity to Ben so Ben does the same.
Heatblast runs into Batman after stopping a fire and he laughs when batman tries to intimidate him as he was facing way scarier foes at 10 years old.
Batman tries to stop Heatblast by using a solution that he uses on fire metas but it does nothing to Heatblast, he enjoys the look of shock on the batman's face before flying away laughing all the way.
Other bats run into Ben when he is transformed into different aliens and they all fail.
Ben gets a laugh out of them thinking he is different people in a vigilante group.
But with him having a bit of an extra personality, different voices and looking different he gets why they think that, so he has fun with it pretending they all work for the "Boss" and that the "Boss" is all mighty and powerful.
Jason has a laughing fit when he finds out that Ben is all the vigilantes and that batman has been going crazy to to find out all the identities of the "group" of vigilantes and how he has been going along with it.
He saves a teenage boy who Unknowing to him is Red Robin.
So Tim becomes a bit of a fanboy about the group of vigilantes.
Ben becomes a father figure to Jason (who is 17-19)
Ben even kills the joker after seeing how he effects Jason. Ben also kills other vilians.
Ben Never tells batman his identity, And only cooperates when he is forced to. (so no becoming a son to Bruce)