I think having day and night could either be fantastic or a complete nightmare. If you have mechanical differences between day and night, it adds more depth, but also potentially restricts freedom.
Let's say that, for instance, assassins moved faster and champs had slightly lower vision range at night, just a general mechanic that's thematically motivated. Now there's more depth, because there's an additional mechanic that factors into gameplay. But at the same time, you've alleviated decision making demands by boiling down what you should do to be highly dependent on a single factor.
Honestly, Summoner's Rift as a map is very static and I don't think it's necessary to start adding so many new ideas to it. I would MUCH rather we load up a totally new map with new experimental ideas.
Now I don't want to bring up any Chambers...filled with magma...but you see where this is going.