If you could change one thing about Zelda 2, what would it be?
From a very high level design perspective, where "one thing" actually means "lots and lots of little things," I'd want to reduce the reliance on obscure or untrained mechanics for completing critical path quests. In particular, I'm thinking of searching the table and fountain for the mirror and water respectively, but also things like chopping down forest tiles to find the hidden town, or the lack of information regarding where and how to use the Spell spell. Each of those has the potential to be a shelf moment. Some of the solutions are alluded to either in the game or in the manual, but even these clues are usually vague at best. I suspect it's this lack of guidance that's given Zelda 2 its reputation as an overly difficult game, as opposed to the moment-to-moment gameplay mechanics, which are in my opinion comparable in difficulty to other more highly regarded NES games.
But in terms of a single concrete thing I'd change, I'd probably cut the fullscreen flashing on death for photosensitive players.