(Update: I have also made a video companion to this article with some of the more interesting theories. You can check it out below!)
10) Runes and Nazca Lines
Let's get the obvious speculation out of the way now, because this is clearly the most tantalizing detail that Game Freak has put into the trailer. In the town with the Grass-type gym, you can see a number of rocks with runes on them, a Stonehenge-like rock structure and a huge mural carved into the countryside which looks similar to real-life Nazca Lines. So what does this all mean? Well the runes remind me of the Unown from Gen 2, but I doubt that they're a direct link with a Pokemon like they were back then. More likely to me is that the runes are simply describing the events of the Nazca Lines that we see. The environmental art here seems to depict a giant dragon-like creature breathing fire or lightning. There really isn't a lot to go off of about what this Pokemon may be like, other than it's large, bipedal and spiky. The art also depicts people and cattle around this Pokemon's feet, but whether they are worshipping the creature or being killed by it is ambiguous (the fact that there is a person lying upside down to the far left of the mural has me thinking that it's likely that this ancient Pokemon was attacking people though).
(Edit: Fiore1300 from Discord has let me know that the Nazca lines as I called them here are called "Hill Figures" in Britain. That doesn't change the implications or the theory too much, but it's worth updating, thanks Fiore1300!)
It's worth taking into account the popular legends of Stonehenge and the Nazca Lines when analyzing the runes, Stonehenge-like structures and Nazca Lines in this trailer. In particular, these structures are commonly associated with aliens contacting ancient humanity in pseudo-science circles. This isn't an unprecedented idea for Pokemon either, as ever since the very first generation there have been several Pokemon which are confirmed to be aliens and others which come from other dimensions. So what could this mean for the game? Well, if this is related to the game's mascot legendary, then perhaps they will be summoned from space by the villains for the game's final confrontation, similar to Deoxys in Omega Ruby and Alpha Sapphire or the emergence of Necrozma in Ultra Sun and Ultra Moon. This seems most likely to me - details like this have pointed towards legendary Pokemon in basically every generation, so the idea that this won't actually lead somewhere is incredibly unlikely.
9) The Themes of the Mascot Legendaries
Pokemon games always release with paired games, but I think pretty much everyone was caught off-guard by how unconventional Sword and Shield were. However, if history is anything to go by, then the titles of these games are going to be a hint at what we can expect out of the games. Since Gen 5, the titles of the games have always tied directly into the mascot legendaries and their themes in some manner - Black and White referred to the dragons Zekrom and Reshiram and their opposed ideals, X and Y were reflected in the names and designs of Xerneas and Yveltal, and Sun and Moon reflected the designs and aesthetic for Solgaleo and Lunala. Therefore, it stands to reason that Sword and Shield is going to describe something about the design and possibly themes of the mascot legendaries for this game. The most obvious speculation is that the Sword legendary will be hyper-offensive, whereas the Shield legendary will be incredibly bulky, which would be quite interesting to see. I think it's also likely that they will both reflect knighthood in some manner, since they are often associated with swords and shields (obviously).
The shared wolf's head in the title design also makes me curious about whether it's meant to be a hint about the legendaries' designs, especially considering how out of place the head is on that shield. Personally I'm thinking it's unlikely that we'd get a hint that obvious, but it is possible that this could signify that the mascot legendaries are a branching evolution like Lunala and Solgaleo were in Gen 7. Also, if the wolf's head is a hint about the mascot legendaries, then it is inconsistent with the dragon-like beast in the Nazca Lines, meaning that that might be another legendary Pokemon in the game. Perhaps the mascot legendaries fought back against the dragon-like Pokemon and kept it at bay? That would be consistent with the idea of knighthood which is inherent in their themes.
8) Could Beauty Contests Be Making a Return?
Okay, I'll admit that this is easily the most crackpot theory I've got here, but I find it incredibly intriguing. So, as we know, professionally-made trailers are always put together very deliberately. Therefore, I find it interesting how brazenly Game Freak put the above advertisement on display in the trailer. At first I just assumed that it was a bit of background decoration to make the world look more interesting, and it's definitely possible that that is all that this is meant to signify. However, if it was put in there as a hint, I decided to check out what each of the berries in the poster was for. I see a Cheri, Pecha, Wepear, Lum, Aspear and Chople berries for sure in that image, but there are also a couple curious details. For one thing, that pointy, red berry appears to be a Nomel berry, but those are usually coloured yellow, not red. Perhaps this is a new berry which is going to be added in the game? There is also a yellow, leafy berry which appears in the background of the image which appears to be a Pinap berry, which is also interesting because this berry has been mainly used recently in the Let's Go games and Pokemon Go in order to make Pokemon drop more candies and level up or evolve your Pokemon faster. However, Pinap berries were originally introduced to be used in the creation of Pokeblocks/Poffins, which were essential for the Pokemon Contests minigame in Gen 3 and 4. Also contributing to this is that the Wepear and Nomel berries which appear in the poster were also used exclusively for Pokeblocks and Poffins. The product that they're advertising also appears to be some sort of Pokeblock treat, which makes me wonder if perhaps this is a signifier that Pokemon Contests are going to be making their return in Pokemon Sword and Shield. Again, I could be looking into this waaaay too deeply, but I really have to wonder why Game Freak would put such a conspicuously Contest-related poster into this trailer if not to hint at something.