I don't see why not to have a mix of special and general issues. Certainly the editors will think of some topics and will receive some submissions that would be worth publishing but might not fit any special issue. Special issues offer a chance to focus and can become stand-alone volumes when done right, but they can also become an artificial restriction, and the concept can fail when articles that don't really fit the special topic are sandwiched in, because you've cut off the possibility of open topic issues.