As of the time of writing, there have only been three new Pokémon revealed for Pokémon Sword & Shield, which are the three starters. There is still no indication of what the cover Legendary Pokémon will be, or how many new Pokémon will be introduced in this new generation of games.
There is an expectation among the fans that at least one new Eeveelution (the term used for the eight current evolutions of Eevee) will be arriving in Pokémon Sword & Shield. The reason for this is that a new Eeveelution has been introduced in every even-numbered Pokémon game. Pokémon Red & Blue introduced Eevee and the first three Eeveelutions, with Pokémon Gold & Silver adding Espeon and Umbreon to the series. The leaked Space World '97 demo of Pokémon Gold & Silver revealed that Leafeon was also planned to appear in the game, but it was cut until the fourth generation. Pokémon Diamond & Pearl introduced Leafeon and Glaceon to the series, while Pokémon X & Y (the sixth generation of games) introduced Sylveon to the franchise.
The Eeveelutions currently cover eight of the Pokémon types, with Eevee itself covering the Normal-type. There are eighteen Pokémon types in total, which means that there are still nine potential Eeveelutions that have yet to be introduced to the series. Pokémon Sword & Shield will be the first games of the eighth generation of Pokémon titles, which means that the time is right for a new Eeveelution or three. The question now is what kind of Eeveelutions should there be in the Galar region?
The most obvious choice for an Eeveelution in Pokémon Sword & Shield would be a Steel-type Pokémon, due to the fact that the games are named after metal weapons and armor, and there have been trademarks concerning something called "Armored Evolution," which seems to be a theme running through the game.
The Galar region appears to be based on the United Kingdom, and knights are very much a part of British history. If Armored Evolutions are indeed real, then it seems that there will be a knightly theme running through Pokémon Sword & Shield, in case the name wasn't enough of a clue, and the next Eeveelution could fit into that.
The Steel-type Eeveelution would likely be called something like Metaleon or Steeleon and would either be made of metal or would have silvery-grey fur that ends in sharp tips, like a blade.
The Steel-type Eeveelution could also have a rivalry with Sylveon, which is the last Eeveelution that was introduced to the Pokémon series, as Steel-type moves are super effective against Fairy-type Pokémon. Sylveon would also have an amazing advantage against our next theoretical Eeveelution...
If Sylveon is going to be facing opposition from a Steel-type Eeveelution, then it only makes sense to introduce one against which it would have an overwhelming advantage - a Dragon-type Eeveelution.
Dragons are an important part of British folklore, to the point where one appears on the Welsh flag, the patron saint of England is Saint George, who was famous for slaying a dragon with a sword, and dragons have appeared on the heraldry of noblemen and on the flags of counties like Somerset.
So, yeah, there will probably be some Dragon-type Pokémon in Pokémon Sword & Shield.
The obvious name for a Dragon-type Eeveelution could be Drageon, but that might be a little too on the nose, so something like Draceon (a popular fan name) could be used instead. The Dragon-type Eeveelution would likely have a more lizard-like appearance than the other Eeveelutions and may eschew fur for scales.
The idea of there being a Dragon-type or Steel-type Eeveelution in Pokémon Sword & Shield almost sounds too obvious with the region being based on the United Kingdom. A scaled Eevee fighting a little knight Eevee would certainly be adorable, but the fans would see them coming a mile off.
Game Freak should buck expectations and add the Eeveelution that fans have the most questions about - the Flying-type Eeveelution.
Eevee and its evolutions are all basically dogs with different colored fur, which is what would make a Flying-type Eeveelution so intriguing. Would a Flying-type Eeveelution have wings, or would it grow huge ears that act like wings, like Dumbo?
There has yet to be a Pegasus Pokémon, but that seems like the kind of thing that would be saved for a Legendary, rather than being wasted on an Eeveelution, so it seems more likely that it would fly using its ears.
The name for the Flying-type Eeveelution is another puzzle, as it would depend a lot on whether Eevee took on bird-like traits or flew through some other method. The lazy answer would be Flyeon, which might be too similar to Flygon.
Eevee was the star of its own game in Pokémon! Let's Go Eevee, so it would make sense for Game Freak to shed some more light on the Pokémon following his recent star turn, so the chances of there being new Eeveelutions in Pokémon Sword & Shield are high. The only question now is what form they will take?