actually when greg berlanti was offered the job before he took it he made them write in his contract that the show would be allowed to be in the arrowverse and have crossovers with arrow and flash. since cbs owns half of the cw theyre sister networks so it wasnt that hard to make the deal work.
also i doubt it will be raven or nightwing, because titans on tnt will have dick grayson as nightwing, along with raven, starfire, a crippled but not called oracle barbara gordon and hawk and dove.
tnt is also a sister network since its owned by time warner, who owns the other half of cw so its possible they could make it part of the arrowverse however none of the arrow/flash/supergirl people are involved in its production so its less likely at this point since so far all the arrowverse stuff have involved the same group of people
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im gonna make my 3 wishlist for the spinoff unrevealed heroes. some sources online are saying that when they said never seen on tv yet they meant never seen on flash/arrow. so im gonna just work with who hasnt been on flash/arrow yet.
1. Zatanna.
shes awesome nuff said. and the arrow folks earlier this year when it was speculated she would show up in an arrow episode said theyd love to use her character in the future
2. Booster gold.
super douche from the future? why the hell not. booster gold is awesome, i love arsehole heroes. were in a world where wow just added in selfies, you gonna try to tell me a dude from the future wouldnt use his future knowledge and tech to make him seem like the coolest hero ever? imagine booster gold taking "past selfies" of his heroics while slowly getting in over his head and actually becoming a cool guy.
3. ms martian.
she has a cool set of powers, and shes a little bit more alien then supergirl is, i dont think the arrow guys would be allowed to use martian manhunter since dc probably has plans for him so she would be a good replacement like supergirl is for superman.
excluded but hope they might be in: jamie reyes/blue beetle, static shock, red robin. we know they cant use the blue beetle because they planned on having blue beetle instead of atom this season and dc said no, static is getting a live action online series so he probably wont be on tv, and red robin is too connected to batman to be able to be on the show probably.
also its possible one of the characters on the team could be a live action vixen