I really wouldn't sleep on the Inklings inclusion being a DLC priority though... Let's be honest here, Nintendo's running on the Wii U really forced them to take some serious L's and right now I think they're trying to gain as much momentum their current money makers before the NX becomes a thing next year or so, and that results in them making DLC to games l that much more important. A Smash game comes out about once every 6 or so years give or take... and that's considering there were no DLC plans for the past 3 games. The way they've planned the game to have DLC in order to maximize it's longevity with content means that they'll most likely look at what would bring them the most sales, and that would be putting in content people en masse would enjoy. Nintendo this console gen hasn't had any real strong hits, but they were first known of fan interest in Splatoon since day one E3 2014, and now we're some months after the game's release and Splatoon's hype was confirmed for real as it's their biggest hit by far on the console next to the Mario games & Sm4sh, breaking sales records and the like. They're new, but they've been front and center for a long time now, their game quite successful, and by now, they're a pretty vital part to Nintendo's game offerings right now. The Inkling Girl had a trophy added into Sm4sh as DLC... If you ask me, considering their game came out AFTER Sm4sh, and they managed to get representation close to a year after Sm4sh's release shows Nintendo's confidence in it now as a series.
Now, if I were Nintendo, I'd probably realize that since this is Sakurai's 'supposed' last Smash, they'll try to take that chance to add the Inklings simply because even though they're indeed 'fresh' to the world, the game's been the one great release they've done on the Wii U and there is a big demand for those Squid Kids, and the Gunner Mii outfits ain't 'cutting it'. Adding them in would also make bank for them leading into the new year because also, Nintendo's plans outside of DLC are practically next to null, especially with Star Fox Zero being delayed until 2016. It's the weakest they've looked in years content wise, so a business move like that would save them some face.
I personally again would like to note that I do see them missing Smash DLC like how Asura from Asura's Wrath didn't get into Ultimate Marvel vs. Capcom 3... but the difference between the two games is that Nintendo's way more committed to offering fan-service than Capcom was back in 2012, based on things like the Ballot and the DLC they did give out in Mewtwo, Lucas & Roy. I'm just saying, don't be surprised if the Inklings do become DLC fighters and keep an open head about them. They've got some strong pull already, and I really don't think 'series longevity' means much anymore considering we got Shulk and Xenoblade representation in the first place, and that game was not only recent (2010) but also niche and low-key until recent years.