With the success of the Arrow/Flash crossovers and confirmation of more crossovers between those two shows, its no wonder there are rumors of other crossovers in the works.
Sadly, Gotham has been branched out on its own, so there is almost no chance of seeing a crossover between Gotham and the up and coming shows of the DCU.
The much anticipated Supergirl is building steam and looks to be on the right tracks with Greg Berlanti heading this one as well. Considering Supergirl's part in the DC Universe is a very large one, we could easily see her pop up on the Flash, Arrow, or the new Titans. The only trouble as Berlanti stated is trying to make the crossovers work and make sense. Super girl cant just fly in knock some guys out and then leave so that could take some work.
It's been years now that the Teen Titans have been in the making and now were finally getting them! The possibilities for crossovers are endless considering the numerous characters involved in Titans. A big one many are speculating about is Colton Haynes as Arsenal crossing over to Titans. Now the timeline wouldn't completely make sense but I believe something could be done sense Arsenal was/is a part of the Teen Titans.
As stated Gotham is almost out of the question considering all the story line in Gotham takes place before any of the Titans are even born.
There are many more crossovers and new shows possibly on the way!