Let's try not to go too wild in over-analyzing what tweets from WayForward or statements by Sakurai might mean for DLC. The fact is that we know very little, and the information we can glean from either of those sources is very limited.
Matt Bozon's tweets, for instance, are very interesting. We can conclude that Shantae hasn't been ruled out for DLC
as far as Matt knows, and that WayForward is not under any agreement with Nintendo that would forbid them from asking fans to support Shantae for DLC.
This probably indicates that Shantae has not at this point been decided upon as a DLC character. Even that is not certain, but if she were already in the works, I suspect WayForward would keep mum on the subject.
Attempting to read anything more into his tweets beyond that is pointless. Matt Bozon himself likely knows next to nothing about the DLC process for Smash Bros. The 4 out of 5 thing was clearly just a silly joke, since obviously no single character or franchise is going to get 4 out of the 5 DLC spots.
The same applies to Sakurai. He states that they haven't yet created any of the paid DLC, and that work will begin on them as soon as they are finished with the main game. Even taking Sakurai at his word, this doesn't indicate whether or not they have selected characters for DLC. And frankly, it's very possible that Sakurai isn't being 100% truthful with some of his statements.
I hate to say it, but the best information we have on Smash DLC is leakers, and that info is hardly comforting. As I saw someone on Reddit point out, a very realistic worst case scenario might be Steve, Dragon Quest rep, gacha character, new FE sword user, and new Pokémon.
The DLC characters might not all be selected yet, but given the relatively quick timeframe, it seems like there's a good chance that they are. As for whether there will be more than one season of DLC, nobody has anything but pure guesses.
What do you guys suggest we Isaac bros do from here? We seem to be back and forth with assist trophy promotion and I think our best course would be to rally for Matthew or Felix to get in as DLC.
I think you should all just support whichever Golden Sun character you most want to join the roster. But, if you are asking from a strategic perspective which of them I think is the most viable candidate, it's still Isaac. He is still very much the face of the series, and the one GS character everyone's been hearing about. The Isaac movement actually has been very successful, in that he's one of the most popular requests now. If you try to switch focus to a different character instead, I think you'll just lose all that momentum. Plus, I still don't buy this AT de-confirmation thing everyone's talking about. The only case in which I think it's de-confirmation is if there's only one season of DLC and those characters are already chosen, in which case we're all just spinning our wheels anyway.
Aside from sticking to Isaac, the next most helpful initiative for the Isaac community would be fan art.
Isaac isn't a controversial candidate; you see very little Isaac hate out there. Rather, people think of Isaac as being an otherwise worthy candidate who is shafted by Golden Sun being a dead series. The way to counter this impression is NOT through argument. When you tell people that Golden Sun isn't THAT dead, or that it COULD come back to life, or that it doesn't matter for Smash if Golden Sun is dead, well, that just keeps the focus on Golden Sun being dead.
What you need instead is for Isaac to feel alive to people. Get people invested in Isaac and Golden Sun. For that to happen, they need to see more of Isaac. That's not going to happen through people playing through Golden Sun in 2018. It's going to happen through fan art.
Fan art has a lot of power, because it gives people a way to connect with a character they don't already know through playing the game. With good fan art, you can get people thinking of Isaac as a living, breathing personality-filled character rather than just another JRPG fossil. Make fan art of Isaac interacting with Smash characters, maybe even in the World of Light. Make fan art showing him annoyed at being an AT again (or hopeful and ready to become more). Use fan art to highlight his abilities as an earth adept, so that people don't think of him as another generic swordsman. And of course, share that fan art where it will be appreciated by Smash fans and Nintendo fans. Twitter and the Reddit for starters. The timing is perfect, because for now at least, Isaac has the Smash community's attention, thanks to the Grinch leak.
Unfortunately that advice is only useful if you have some talent for fan art. It wouldn't do me any good, for instance. But the Isaac community is big. If you can get Isaac fans showing off their artistic chops, or, failing that, commissioning some cool works, I think that's the most you can do as a community.
On a less positive note, Isaac fans need to keep their expectations in check, just like all other fans. For all we know, all the DLC for Smash Ultimate may have already been decided upon anyway. Character campaigning for Ultimate DLC may very well be totally futile. Share the Isaac love just for fun, but don't invest effort and emotion into campaigning that wouldn't be worth it to you for its own sake.