Splatoon seems to be developing quite the fan art following already....

the pics are pretty big from pixiv so quoting to make them small.
Looks like you tried to link images directly from pixiv. Understandable so you're not reuploading the images, but this sadly doesn't work for anyone who isn't currently logged into Pixiv right now. There is an embed function that works with HTML Code, but I don't know how well it functions with GAF.

Also all this art and nothing from sameanko? For shame!

Source: Sameanko's Twitter

Source: Sameanko's Twitter
Note: This image is a parody of Nintendo Japan's Online web store error message (Thanks Nibel!)

Source: Sameanko's Twitter
Rough translation: "Hurry up and come Summer! I want to play Splatoon 2!"
What I want to know: How are they going to do Splatfest concerts if Callie is abducted in Hero Mode? How would that work for players who are attending Splatfest but haven't "rescued" Callie yet in the campaign?
For the fact that Marie seems to be spending her time in Cuttlefish's old spot loitering over the grate (and the fact that they're apparently unrecognizable to the new squid generation), my assumption would be that if Splatfests still occur, the squid sisters no longer preside over them regardless. Once reunited, you can probably just catch them hanging out in Octo Valley or maybe hanging out in the square to serve some other "postgame" purpose.

It's also obvious by now that they're no longer presiding over news updates, so how they opt to relay that information now will be interesting to see as well. Very few characters in this game seem to actually be directly taking on the same roles they used to have, and I kinda like it.
(and the fact that they're apparently unrecognizable to the new squid generation)
Incorrect, Squid Sister Stories establishes the end of chapter 7 as very near the beginning of Splatoon 2 and that Marie had to find Callie quick enough for others not to question her absence. Other supplementary info indicates that only Agent 4 (your character) does not know the Squid Sisters, is new to the area, and probably doesn't watch a lot of tv.