the consideration that Smash always ends in a not big note doesn't click so much with me. I mean, there is a big difference between base game characters announcements and dlc characters reveals. So the fact they ended with Shulk and Incineroar doesn't matter so much. But even in this case, I'll say something later in this post.
I think we should look more at the last dlc announcements: for Smash 4 they ended with Bayonetta; Bayonetta isn't a super big, over hype inducer character, but still was a surprise character (almost mind-blowing, I'd say), with more than decent support and she won (more or less) the ballot, so she was a very requested character by definition.
For Smash Ultimate FP1 they ended with Byleth, of course, but they revealed Byleth only AFTER they announced new dlcs were going to come; so that wasn't an end at all, and it's clear that they already knew that almost certainly there would have been a second FP after the first one (only if the first one and Ultimate in general bumped very hard in terms of sales, an FP2 wouldn't have been worked on).
These aren't enough data to say anything about how much big the last dlc will be, so we can dismiss (in this moment, at least) any fan theories based upon alleged past tendencies in Smash history, and bound ourselves to the logic alone. The logic I think would say anyone that the best thing to do in order to convey hype is to end with a veeery big, unexpected, maybe mind-blowing character. Unfortunately I don't think that character could be Dixie.
Of course though, never say never! Also because, the difference with Smash4 dlc is that Ultimate dlc are selled mainly as pack, so in order to sell a pack it can be useful to reveal the biggest characters earlier, and the less hype inducing after. So, ending with a Nintendo character? after the budget to buy guest stars to sell the pass is already spent. Yes, it's possible... but I guess that in this case they could choose to end with a pokèmon character: maybe a pokèmon character doesn't bring much hype, but for sure everyone know pokèmon, it has many many fans and so it's likelya pokèmon character would sell enough. So, I see it as a good final character. We shouldn't forget also (and now I arrive in the point I alluded to earlier in this post) that for Smash Ultimate base game they ended with the reveal of Incineroar (and Ken): I don't think they considered this ending on a low note. Of course it's not so high like a guest star or a Ridley/K Rool, but still they knew that it, since is a pokèmon, would have made happy many people. There is who thinks that Incineroar isn't the best pokèmon they could choose for this, because not the most popular one, and that for this reason Sakurai announced it for last (so, "ending on a low note"), but I don't think it's true: if they thought it wasn't a hype character, they would have just chosen another pokèmon. Or am I wrong?
So, tl;dr: I don't think they necessarily will end on a low note, even if it's likely the last character won't be the most megaton hype character in the pass (and so maybe a first party character, or a relatively smaller guest star), but still a character with many fans and that can sell much. It definitely can be a pokémon again (but as I said yesterday, SWSH is already old news; the next generation is still too far away; and they never choosed from a remake until now).
OR it can be another Nintendo advertising character, and in this case I could get very mad if it won't be Dixie this time either!