See the relevant Stack Overflow meta post.
I think this is a good suggestion. I would support it, but let's give it a few days to garner support in meta.
I would propose that we follow the convention that StackOverflow has taken like you suggested:
- create unity-container tag just like SO's.
- Audit the current unity questions again to ensure that no unity3d posts remain.
- merge all remaining posts in unity in to unity-container
- create alias from unity in to unity3d