3 raid tiers and a strong possibility of a short filler raid
Vanilla had 7 tiers:
Burning Crusade had 9 tiers:
Wrath of the Lich king had 8:
Eye of Eternity
Trial of the Crusader
Cataclysm had 6:
Bastion of Twilight
Throne of the Four Winds
Pandaland (will) have 5:
But that's not how tiers work. There can be multiple raids along a parallel tier or half tier (AQ 40 was 2.5, BC had 3 dungeons on t4, 2 t5, 3 t6)
1. No matter how you view it, the last raiding tier of an expansion has to last somewhat more than previous tiers during the expansion, since it encompasses both the normal duration of the tier and the duration of the period where the tier is nerfed for some guilds to catch up. Right now there are guilds progressing in tier 14 in its nerfed state, while the others progress in tier 15. Come tier 16 some guilds will be progressing in tier 16 content while others will be finishing up tier 15. But in tier 16 there is no following tier to allow guilds to catch up. So the logical thing to do would be to add a period of time where tier 16 gets nerfed and let guilds that need the nerfs play. This is also hinted with the inclusion of the (in)famous unannounced special feature of Mists: being able to run older content as if it is current somehow. Why would they give players such a massive amount of content but not the time to enjoy it? Most probably in my opinion tier 16 will be extended so that guilds that need help in progressing in it will get it in the form of nerfs, while most of everyone else will play with the new special feature.
2. No matter how fast the developers work, there is no way to release a new expansion as fast as the current tier cycle makes some people think. A new expansion is a massive amount of content compared to patches, even major ones. Just testing would take a long time to carry out somewhat succesfully, and we don't even know anything about the expansion, much less have a beta testing date. Also the fact that Blizzcon is held in autumn and not the summer, and so late in autumn actually, where the new expansion is almost certainly going to be revealed, leads me to think we have a long way to go ahead of us before any 6.0s arrive.
We'll have 3 tiers, which I have no problem with.
The tiers so far have been quite enjoyable.
I think Siege will be just as epic as ToT currently is.
Unless they push out the next expansion faster than ever before, we're still looking at a near 1 year gap between SoO and the 2 year mark when they usually release the next expansion. A bridging patch can't just be a scourge invasion or even the Halion patch. It would need to be bigger in scope with a good amount of content to stave off that horrible year of nothing. Or of course get the next expansion out faster.
I believe a MOP will have 3 raid tiers total.
5.0/5.1 - MSV/TOES/HOF
5.2/5.3 - TOT
5.4/5.5 - SiegeOfOrg
IMHO we'll get 5.5 but not raid after SoG