It is very difficult to make blanket suggestions in terms of "what to buy next", as many factors enter in.
In many ways, the "best" purchase will depend on your writing style(s), whether you are interested in writing traditional classical and or chamber music, or more towards the film scoring side of things.
As Paul said, do take a look at VI Pro. As VI Pro includes the "stretch" fuction, it is possible to create faster legatos by using the stretch function to create new, modified, legato patches.
If you are interested in building your basic orchestral sections, the orchestra strings libraries, and woodwinds I would be a good place to start in terms of adding the full collections. As the fast legatos are in the extended portion of the full libraries, you would need to buy full collections. Of course, if you could afford the much larger collections, that would be more cost effective in the long run.
Another common approach among VSL users, is to get AP Strings as the first full library to go with previously purchased SE libraries. Again, though, it depends on your desired writing style(s).