Along the lines of added/improved dialogue, I wish that every or
almost every NPC could have lines that address the player's wearing of the Amulet of Mara and flirtation/proposal - for better
or for worse, instead of sometimes not at all (as we know, many NPCs totally ignore the Amulet).
So for example, if you wore it around Brynjolf or Teldryn Sero, even though they're not marriageable vanilla-wise, they could have lines to indicate that aversion to the vows. Like, "You're sweet, but...I'm married to my job" or "It's not you, it's
me". I think it would have added yet more realism and flavor to the whole marrying thing, instead of it seeming like a mere game mechanic with little charm or even utility in some cases.
I realize this would have required more dialogue for the "generalized" voices as well as the unique ones, but I think it would have been well worth it. Could have been very amusing, anyway.