I know there is something about the theories on the Colliseum event as a thread and there is possibly some whine thread that I cba to look for it.
What I think is that since the Colliseum currently does not feel very 'lore like' due to Blizzard said: every major patch we want to expand on the Arthas storyline, and let's face it, this currently does not have any major characters that we know of lined up, however, as stated in the theories thread of what could happen, there is potential... What I think as my own theory is that the Colliseum will be just that Ring of Blood concept of maybe 5 bosses which maybe 4 are random of maybe a pool of 9 bosses and then the 5th and final boss will always be the same and something maybe big in Northrend lore.
Now what I really wanted to add in mostly, is that I think Blizzard still has another secret raid instance lined up and they only thrown this at us to stop us from guessing, because come on let's face it... sure this was 'secret' at the time, not many people guessed that the Argent Tournament would be in fact a raid instance that fills up the whole patch, nearly everyone that did guess pretty much thought it would be maybe a quick encounter like EoE.
I believe, and maybe others are thinking this too, that there will be another raid instance that will be the BIG news that will be in this patch, that will really carry the storyline, or possibly bring us back to the old world. Think of 2 instances you can both progress in at the same time: SSC and TK, Hyjal and BT. Where as I see the Tournament lore is simply getting the 'champions' ready to take on Arthas and Icecrown, but the Argent Tournament can only really bring in bosses they captured or whatever in the outside world or personally trained, unless as already mentioned in the theory thread by 1 person (since I last checked it) was that Arthas appears and turns the instance into a Nerubian tunnel that connects to another instance, which is cool.
So for those TLDR people: I am guessing that the Colliseum is only part of the next major patch and there will be another big 'lore' instance to be announced in a couple of months, maybe Blizzcon.