Custom towns/Town editor. Afterall, you will end up being the mayor in AC3DS, so why not be able to shape it how you want?
Placeable paths, which could also work in a town editor, since the path ideas were good in CF, but it was so tedious to make them and eventually you would end up wanting them gone.
More things for the town as well, like the lighthouse/fountain/windmill etc, and placeable bridges.
anything else I can't really think of right now, but a tools bag like it's been mentioned before would be perfect, a fenced garden space outside the house, and more fruit would be nice too.