It is very common for new games to be first teased with a cinematic. Recent examples are bayonetta 3 and Travis strikes again. It is not very common for ports to be teased with just a cg trailer. Every port in that direct was announced as a port and had gameplay.

Not showing gameplay adds to speculation, it gets people talking more. Likely they are waiting till e3 to show gameplay. They are building anticipation for e3.

Mario Kart 8 Deluxe was teased with gameplay footage several times before being announced, and when it was announced it was clear it was a port. Pokken was clearly a port from the start, the same with Donkey Kong, Hyrule Warriors, and Captain Toad. Whatever is going on, they are treating this differently than literally every port announced so far.

What was the last game that Nintendo announced less than a year before release without any sort of title whatsoever? What was the last Smash game that started development immediately after the last one? What was the last Smash game that was without gameplay shown, or any sort of trailer besides a blacked out version of characters, this soon before release? Hell, what was the last game Nintendo announced that they didn't quickly confirm was a new game?

Regardless of whether this is a port or a new game, we're dealing with uncharted waters here. Even SM3DW was announced nearly a full year before release, in a January Nintendo Direct, and that was a relatively low key 3D Mario game compared to something like Galaxy or Odyssey. Announcing a brand new Smash with even less lead in time makes no sense at all, given that a Mario game has one major character to market, Mario, and Smash has literal dozens.

It was only two months ago that Dark Souls' announcement trailer for Switch dropped with no gameplay, yes? I'll grant that it may not be frequent, but it's certainly not out of the realm of possibility.

I'd buy the speculation argument if we weren't so close to launch. If you wait until E3 to reveal gameplay, you have 5 months at most to show off every single new character that you want to display before launch. You'll have to pack in so many trailers in quick succession you risk overloading audience attention and having some characters forgotten or ignored due to the announcement of others, or alternatively not advertise them at all. There's a reason Brawl and Smash 4 chose to draw out the character reveal process over a year plus, and Smash 5 choosing to buck history and try to compact all of that inside five months doesn't make a lot of sense.

They may not care to draw out new character reveals for so long this time and handle it more like Melee, where it was announced at E3 in June and released that November, about 6 months later. At this point I doubt they are adding so many new characters compared to previous games, and if they are bringing back single player they may want to emphasize that instead.

My guess is this will be similar to Splatoon 2 to Splatoon 1. A "new game" but one that is built upon the foundation of the previous game and even recycles some of the same stages.

A lot of people here were saying Splatoon 2 for 2017 would be impossible, it would have to be a port of Splatoon 1 because 2 years wasn't enough turn around time for a full sequel, but it was a new game.

My guess is they'll take the Smash 4 models and refresh/update all of them, tweak the gameplay, add a full blown single player story mode, and all-new stages/music with some Smash 4 stages returning. Add in new characters (Inklings from Splatoon, ARMS characters, Solid Snake, Simon Belmont?).

They said from the start that dark souls was a port. They didn't just drop a cg trailer and say nothing.

There isn't much point in comparing Nintendo's past marketing of smash, there marketing is just so different now. This may be more like the marketing of Pokemon. Which for the last few games has had announcement trailer early in the year and is then marketed throughout that year. Pokemon games have potentially over 100 characters to advertise with each game.

Looking at a lot of recent announcements the gap between the first teaser and the game coming out has been pretty small. Kirby announced at June 2017, released March 2018. Fire emblem warriors January 2017, released September 2017. Xenoblade 2 announced January 2017, released December 2017. 

The problem with showing off smash too early is that it could distract a lot from other games. Just look at this direct, I've barely heard a word about anything but smash. 

Unless a game is stated as a port there is absolutely no reason to believe it is a port.

Even Smash 4 was a lot like that.  Yes, it used a new engine, but it had a ton of stages that were recycled from previous games.

I would bet on some reused stage geometry.  Wii U did that too iirc.  But new effects, lighting, prop detail.  Character models will likely be almost entirely new though.  A few may be retextured Wii U geometry though, we'll see.  

But maybe not, we'll have to see.

We're shifting ground a bit here, going from "no ports are announced without gameplay" to "no ports are announced without stating they're ports," but all right. Regardless, there aren't a ton of instances where a company drops a trailer for a new game without stating that it's a new title, either.

I can agree with the notion of "unless it's stated as a port, there's no reason to believe it is a port," if you only look at the way a game is announced as your basis for determination. If this trailer had dropped and said "releasing 2019," then I would fully expect a new game as well. But you can't look exclusively at the way a trailer is framed to determine whether it's a game or not, which leads us into the marketing points.

I can fully agree with smaller titles that often having difficulty maintaining interest over long spans of development time, especially Warriors. But even titles like Xenoblade give themselves generally at least a year to market before releasing in the hopes they can build up interest, and the same generally applies to Nintendo's larger/well recognized IPs. Mario Odyssey, Breath of the Wild, Metroid Prime 4, Fire Emblem 16, all announced well before their release. In the latter two cases, Nintendo went out of their way to announce the game's existence before even a CGI trailer was available. The only consistent exception I can think of is series that release new installments so frequently it would be next to impossible to advertise the next game with the same run time that games like Smash have had in the past without infringing on the advertising territory of the previous title. Kirby and Pokemon tend to fall into this category, and it's worth noting that even Pokemon may be moving towards a more long term focused advertisement scheme for their Switch game, which, again, Nintendo went out of their way to tell us was coming before a trailer was even ready.

I won't say that there are no examples of big name Nintendo titles announcing themselves very soon to release; Splatoon 2 comes to mind as the most prominent example, but they're so few and far between and such a departure from how Smash has handled itself in the past it makes me very skeptical that this is an entirely new game.

Finally, the idea of Smash distracting. In 2006, Nintendo had Twilight Princess, Metroid Prime 3, Super Mario Galaxy, and Smash Bros. Brawl all announced with trailers shown at the same time, and every single game sold quite well. This year, they had Super Mario Odyssey, Metroid Prime 4, Pokemon Switch, Fire Emblem 16, and Splatoon 2 all announced at the same time. If Nintendo's particularly concerned about the idea of overshadowing smaller games, they certainly haven't shown it in the past.