I like the idea about expanding Riverwood! It is one of the starting areas in the game, and there is always some feeling of familiarity when I return to it. Plus, it has some calm, provincial charm to it. I would suggest to stay away from putting anything "epic" like manors or Dwemer ruins in Riverwood. There are already a lot of them in game. Just keep it simple and stylish. I think a few hunting quests would fit its image fine, like the suggested one with the troll. Also, there is Anise's hut with an old lady (who, well, knows a few tricks). You could involve her in some local quest. Actually, the whole area between Riverwood and Falkreath is very full with small, but interesting places: cave with wolves, Talos shrine, burned cabin. Finding one of those could start a small quest as well.