I felt similarly when watching the trailer. Though that may be due to me not having played Three Houses yet.
But watching the trailer, the reveal portion seemed less creative than other recent Smash character announcement. Joining Smash gets you out of a dark dimension?
But I liked the self-aware joke about having too many swordsmen in the game. That part felt up to par with the other trailers.

I enjoyed the showcase of the character though. Looks like a character I'd use.

I don't think he meant that new is necessarily a barrier in that regard. But rather that because he wasn't chosen for the DLC fighter pass, and is new, the window for when Rex would even be under consideration is more limited. When fans started requesting him, they may have had to wait until Fighter Pass 2 was greenlit before discussing his inclusion. So Sakurai added a Mii costume instead, because that wait would be long.

If Byleth had not been chosen for Fighter Pass 1, it may have also taken a while for him/her to get considered. But because Three Houses released closer to when discussion over who would make the Fighter Pass 2 shortlist began, Byleth fans may not have needed to wait as long as Rex fans to see him/her in the game.

I figured it was a reference to something similar that happens in the game. Especially with the old man's comment. But I tried to imagine if I'd find the Smash invitation a clever solution to the problem, or if it would make sense. And that is hard to imagine. But maybe it was.

I can get excited about guest character trailers from games I haven't played. But it can certainly help if I have.
I thought the way they introduced Banjo was great for example.

Personally, my only complaint is that it wasn't Edelgard or Dimitri (Claude would be a bit tough as a mounted bow character). Three Houses was my favorite game of 2019 and I suspect the characters will go down as among the most memorable of the whole series. As such, it will age fairly well. Again, though, I would have gone with one of the House Leaders since they are more memorable than your mostly-silent avatar and have a whole lot more personality behind them.

In any case, if you were surprised that we got a Three Houses character after it's already been announced that there would be DLC beyond the first pack, then I think you were fooling yourself. I was a bit surprised we got it here, but I would have been more surprised if we never got it by the end of the DLC offerings. It was just a matter of time.

Here's a suggestion that would have been awesome.  Since in Three Houses any character can use any weapon, they could have made it so that they all were playable using mostly the same moveset, but each character uses their own relic.  Say, Edelgard has a silver sword, but Byleth has the Sword of the Creator with its own special moveset.  And so on with the other relics.

You could further differentiate the characters by their weaknesses.  For example, Edelgard's Nair and neutral special could be a bit nerfed since she's weak with bows.  Claude would be poor with lances and Dimitri would be weak with an ax.

It would make for an interesting take.  They wouldn't be echo characters, but they'd be pretty close with a few key differences.

Was the Byleth situation really like that? I thought Corrin's choice was clever and coherent because it was presented as an actual in game mechanic alongside 3 other choices in 3DS graphics. And they were all about considering invitations to join a side.
Even if I hadn't been familiar with that particular in-game scenario (and I was), I still would have understood that you'll be presented with that list of choices of who to join at some point in the game. So I thought that was pretty creative.

K.Rool's reveal makes more sense if you're aware of the fan requests. But even without that context, the troll from Dedede was pretty funny.

The SNK characters chasing the invitation letter had references that people who played those games would appreciate more.
But I think anyone watching can appreciate the characters chasing the invitation as a creative little storyline. They all desperately want to get invited to Smash, but get snubbed one by one. And the way the trailer begins with NEO GEO graphics reading like an invitation letter, even if you haven't played any of those games you can understand that at least one of those games has an invitation scene like that.

From watching this trailer though, they presented a problem for Byleth. It was obvious that something similar probably happens in game.
But I didn't get an indication that the solution to whatever situation that was in the actual game had any similarity to joining Smash.
Maybe if I play the game, it turns out that Byleth's way out of there is to join some group. Or maybe he'll use an object similar to that envelope. But I got the sense that that's not the case, and that the invitation felt out of place here unlike the previously mentioned trailers.

It's not bad if that's the case though. You can't always find clever ways to intertwine different works of art that way.

When you say that, it sounds like Sothis (in Three Houses) suggested an irrelevant solution to the problem, that wouldn't actually help Byleth get out of there. At least at first, before an actual solution is presented. But yeah, don't tell me if it could spoil anything.
I didn't consider it could have been a joke like that. But as long as it references the game in some way, then it's clever and consistent with previous trailers. I thought it may have been completely random even if you had played the game. Which I'll take. It just wouldn't be as good as some of the other trailers for me.

The characters were already decided before the pass even released this was stated when Joker was revealed much like he said about the second pass, a TH rep was always on the way as the characters lend themselves well to a fighting game.

I like Rex but I rather more series get represented than more characters from a series.  One of the few exceptions I think would be Final Fantasy. I'd love to have Tera. I love how DQ did it. One character,multiple skins to represent the different heroes in DQ history.

To be fair it's entirely possible they changed their picks around and moved the 5th fighter onto the second pack. They sure wouldn't want to start off their fighters pass 2 with a Fire Emblem character.