When WOTC stops giving him work perhaps? I imagine there might be some noncompetitive clauses in his contract.
It hasn't been a problem in the past. Aside from his "Guns of Alkenstar" series, Ed created and introduced monsters for every volume of Kingmaker and took a first stab at the outline for that campaign, kick starting that whole process. But, I believe by his own admission as well, Ed is more a story guy than a hardcore rules guy, and I'd much prefer match up people with their strengths and what they're excited about writing than shoehorning them into weird assignments.
That said, if Ed ever rang us and said he'd be interested in writing adventures, we'd be excited to listen.