Blizzard left out really awsome forces in Pre TBC. And yes, if you do make tiers, you should extend into T4, like Asomath said. And a problem I find, is that a lot of the tiers come purely from one instance.
T0.5: ZG only
T1: Molten Core only.
T2: Most from BWL, 1 from Onyxia
T2.5: All from AQ40
T3: Naxxaramous Only
But, if we added some new ones...
T.5: The peices from ZG, added by peices from a new 20 man raid, Uldum
T1: Half is from Molten Core, the other half along with item peices of the set come from a 40 man raid, Stonemother's Sanctum
T2: 1/3 is from BWL, 1/3 is from Onyxia's Lair (stretched out, more bosses) the last 1/3 is from a new 40 man raid, Mount Hyjal
T2.5: Half is from AQ40, the other half is from a new 40 man raid called "Caverns of Time"
T3: Half is from Naxxaramous, the other half is from a raid called "Invasion of the Scourge" (Will explain)
T4: All is from one, very long 40 man raid, (It's kind of split up into 3 raids, but they are techniquely all one raid) Called the Maelstrom
The new raids I mentioned
Uldum
This is not actually all of Uldum, It's the first line of defence in Uldum. The rest is behind a massive gate that no one can open.
"Within these halls, lies the secrets of the Titans. Within these halls, lies your death"
~Furlorn, Watcher of Stone
Stone Mother's Path
This is a Raid in Descolace, which is basicly like Molten core with it's below BRD only Stone Mother's Path is below Maraudon.
"Did my brother's fall? Yes. Will I fall? Never again! Come and fight!"
~The Stonemother (it's one of the Old God's luitenants, like Ragnoros. I forgot the actually name of her, so atm she's just Stonemother

)
Mount Hyjal
Mount Hyjal is a raid where Players travel to the corrupted lands of Mount Hyjal, where archimonde attempted to siphon the powers of the World Tree. In his wake of chaos, lies forgotton horrors...
"No quote"
~Someone without a quote
Caverns of Time
This is not the Caverns of Time 5 man dungeons in TBC and wrath of the lich king. This is the Caverns of Times where a war against the Infinate Dragonflight occurs. You are aided by other members of the fight during the raid.
"You cannot outwit time itself, mortals"
~Unknown Leader of the Infinate Dragon Flight
(The last boss looks "similar" to Nozdormadu, adding to the rumor that he is Nozdormadu, but there is no proof. He is not slain, as much as just runs away)
Invasion of the Scourge
This is not as much as a raid, as much as an event. After players defeat the impossible chalanges of Naxxaramous, they are attuned to this Event. They can start the event and will face the unlimited horrors of the scourge attempt to retake the Citidel.
"This land is mine, not one spark of life shall remain"
~The Lich King
(Again, players see him, but they don't fight him. Infact, at the end of the dungeon, he presents himself, unleashes a boss, and then he watches. After the boss dies, he says)
"Bah... we shall meet again... and be warned... Naxxaramous WILL be retaken!"
Maelstrom
Taking a boat to the Maelstrom, (Kind of like a cinematic, they crash in the tempest of it) They get a debuff which will decrease their stats, you know, cus they're weakened by the tempest. In this dungeon, they will fight foes such as Azhara, queen of the Naga.
"Visitorsss... The lassssst time I had compony wasss... Defend the maelssstrom! Thessse mortalsss shall be consssumed by our power"
~Azhara
Just ideas. I find they kinda gay myself, so yea. Don't add them if they seem gay to you too.
Sorry I don't have any boss fight ideas or anything, it's kinda late :
