What you could add-on depends on the exact nature of your existing business. I assume we're talking about commercial window cleaning, at least primarily. Do you also do residential? Are we talking about high-rise stuff, with the guys on those platforms dangling over the city streets, or something where they wear harnesses? Or are we looking more at "ground level" stuff?
If you don't do residential, you could add that. If you already do residential, add power washing of siding and decks. In fact, with these "deck resurfacing" products that are out now, you could offer deck restoration services. Exterior power washing of walls/sidewalks would also potentially work as a sideline for a commercial oriented business.
Basically, I'd say look at any sort of exterior maintenance / cleaning service: stuff that might require equipment your clients don't have, or that is "icky." People will pay a lot to have someone else take care of the "icky" work for them.