Alright, here's my two cents on the matter (I posted this to Serebii forums, in a similar discussion).
Why we could be seeing Generation VIII in 2018.
Announcement/reveal dates, first news dates and release dates
1. Generation V was announced on April 9th, 2010 and were released on September 18th, 2010, with the games being revealed in January of 2010.
2. Game Freak could be putting the announcement on hold, maybe later this month or sometime in April as they could be reviewing past announcement dates and the time between the dates of the announcement and the day we get our first batch of news. Remember the almost three month wait between Su/Mo's announcement and the first batch of news (February 27, 2016 - May 10, 2016)? Masuda apologized to fans for the lack of news between that time period, so that could mean that they'll announce the games on a later date and keep the release as a holiday release.
"Patterns" and speculation
1. The previous Pokémon Direct (the one announcing Pokémon US/UM) has been deleted from Nintendo's archives and this happened before Pokémon Sun and Moon and Pokémon Ultra Sun and Ultra Moon were announced, this is one of the "signs" and "patterns" fans have been talking about.
2. A few weeks prior to the announcement of Pokémon Sun and Moon, Masuda tweeted a picture of himself composing music, he did that again a few weeks ago (while this might mean nothing, people are talking about this).
Is the end near?
1. According to Pokéshopper, the UB line of merch is ending in April and there is some merch that is yet to be revealed, relating to Eevee, Lugia and possibly Zeraora. And that will give us a pretty big gap between merch and new merch if the game is a 2019 release, maybe they'll release figures/plushies of the main human cast of the games, that'd be good.
2. The Sun and Moon arc is over in the anime and it's now the Ultra Sun and Ultra Moon arc, maybe there'll be some more arcs like Team Rainbow Rocket or URS for example, or maybe an entirely new arc (New Gen VII game?). Surely, they can't have filler between arcs.
Wishful thinking
1. Maybe the Switch game could be the anticipated "Pokémon Stars" a Gen VII game, but it's certainly not a new thing for Game Freak to put a game from the same Generation on the next console up.
Why Generation VIII may be unlikely in 2018
Reality
1. We are still in Generation VII and Generations usually last for three or four years.
2. Like I said, the anime could have more arcs as filler between Gen VII and Gen VIII.
3. Just because the current line of merch is ending, doesn't mean a new line isn't coming.
Game Freak and their curve balls
1. Remember how Pokémon Gold and Silver were both released "for" the Game Boy color, but worked on the original Game Boy? And how Pokémon Crystal was released exclusively on the Game Boy Color? Maybe the Switch game (if it comes out in 2018) could be another Generation VII title (Pokémon Star(s)), it's definitely not a new thing for Game Freak to release games on the next console up.