I don't know, animated Deathstroke always had really confusing motives, I would rather have Deathstroke with clearer motives.
If you adapt some of the key Deathstroke stories from the comics, you have the H.I.V.E be a generic evil organization trying to take over the world or something and they try to Deathstroke to deal with the Titans. Deathstroke refuses, so they hire his son Ravager to take the contract. Ravager dies while fighting the Titans and Deathstroke decides to seek revenge on the Titans and is willing to work for the H.I.V.E and use their resources to seek this end.
So the Titans would have to foil H.I.V.E's scheme, while dealing with Deathstroke at the same time. So the H.I.V.E would technically be the main villains, but Deathstroke would be the villain who moves the plot along more often then not, though the Titans can fight H.I.V.E goons through out the movie.
Then in the second movie, after the H.I.V.E is defeated, Deathstroke returns with another plan to get revenge on the Titans and uses Terra to destroy the Titans from within. I think you would need two movies to tell the story of Deathstroke vs. the Titans.
Also as mentioned by another poster, you have the likes of Jinx, Gizmo, Mammoth and Plasmus be a goon squad for working for the main villain of the movie, playing a similar role that the Brotherhood and the Hellfire Club played in the X-Men movies.
