Sorry, but I missed al these last night, so here you go! Tumblr Pages:
https://splatoonus.tumblr.com/
1) This post confirms what we already know. The Octolings will be playable in Regular and Ranked Battle once you beat the Octo Expansion.
2) Some more info about Cap'n Cuttlefish. It's says that he is the leader of the New Squidbeak Splatoon (which I think most of us know.) and recently he was on patrol with Agent 3. However, he got sucked into this underground world and lost track of Agent 3. Maybe Agent 3 we play a big part in the story.
3) This says what I've basically already said in another post: To move around this underground area, a subway named Deepsea Metro is crucial. The passengers are very different species then what you see above the surface. It also says how they're not bad guys. Maybe we'll get to talk to them?
4) This post shows a stage that you'll be playing. If you remember from the trailer, this is the stage with the Inkjet and Octoblimps flying past. It also say that each stage will present a unique task, and in this case it's clear the stage with only a special weapon.
5) Not related to Octo Expansion, but still interesting nonetheless. Once you meet a condition in 3.0 (and presumably have beat single player.) Callie will appear. What Callie does is she tells you different gameplay facts. In this post, she tells you how many times you open the map in battle, and how long you have it open for. Marie then follows up with some gameplay tips. This does seem like a interesting thing.