I'm really not convinced that the 4 giant guardians are THE dungeons in the game. You'll obviously have to enter them to defeat them, but I think that the insides will be quicker areas that involve more platforming and rune usage. Basically, they'll be more like large shrines then they will fully fledged dungeons.
We get 2 peeks at the inside of a giant guardian (you can tell this because they have the same red particles that are present on the outside of the corrupted guardians), once at
2:26 of the trailer and
another time at 3:01. The structures look much more like something I would expect from shrines, especially with the first example.
This is likely the Death Mountain guardian. To me, it just doesn't look big enough to hold a full fledged dungeon. The camel and bird guardians do look big enough to hold dungeons, but instead I think they'll be filled with simple large rooms like my first example above.
There are 4 confirmed runes so far: bombs, cryonis, ice pillar, and magnesis. I think that each of these guardians will have puzzles that mainly focus on one rune, and the 5th mystery rune will be used to defeat a final guardian that you get to near the end of the game (I personally think that the last guardian will be in Hyrule field, which is where the 5th pillar surrounding Ganon's Castle points).
And with such an enormous world, it would be strange to focus so much of the game's content around a few small areas.
I'm interested to know what other people think about the giant guardians, I'm usually really wrong about with these types of theories.