Format is usually open to voting as well, so it is up to popular vote whether we see Summoner's Rift or ARAM, for example. Those tend to be the two popular choices, I don't recall if Twisted Treeline is usually an option or not but I don't believe it has brought in near a majority in the past if it was. We also have typically conducted both a bring your own team and a random team version in the past. I'm uncertain if we will continue with that at v15.0 however.
Like Garfi3ld said, we should have some more information shortly. In the mean time, please let us know if there are any other questions. Hope to see you there!