I like the idea, especially the idea for many more new weddable NPCs, quests for each, and the Honorhall Orphanage adoptions. However,
Spoiler Mirabelle Ervine is killed by Aincatar
.
This should be DLC I agree. Everyone saying no and that the next DLC should be Thalmor related should be aware that DLCs, while expansions are included in this category, are small additions to the game, usually involving only a medium length questline, perhaps armor and weapons and homes.
Expansions are large DLCs with a main quest comparable in length to the original MQ and side quests and whatnot.
THAT is what the Thalmor Invasion or Thalmor related DLC should be -- an expansion.
The OP's marriage idea should be a small DLC