They have pressure on their team to release on Switch as early as possible, I don't see it coming later than 2019. November 2019 at the latest, no way they wait until 2020 for the Switch pokemon game.
As for remakes or side games, it makes no sense to put resources on those when, again, the pressure is for a new Switch game. Much like Gen 5 didn't give us the RS remake we all expected, and instead they jumped the gun on 3D models in Gen 6 and THEN gave us the remakes. In that same way, I expect Gen 8 on Switch, and then maybe the Gen 4 remake people keep expecting.
The merch cycle is fairly advanced too. The only thing missing in terms of legendaries is Zeraora, or w.e. that thing is called, which we can easily get this year. Or it could even have the Zygarde thing where it got pushed to the next gen.
So we can easily have movies this year that conclude gen 7 while teasing newer mythicals or legendaries from the next generation. The anime is also fairly malleable thanks to the school format it has, so that's not a pressure to delay things either, and I'm sure it's more of a case of the anime having to keep up with the speed of the games and not the other way around. As for VGC and TCG, I don't know much about the cards but don't see how that's a pressure. VGC defaulted to USUM in November, it's not unreasonable to have that meta last this entire year before they want to mix things up with new mechanics or mons.
tl;dr, there's no way next gen waits until 2020. With that scope in mind, it makes perfect sense to assume that certain mons and certain design ideas are down already as they work on the rest of the game. I mean jesus, if they said they'll start work on this at last year's E3 and STILL haven't gotten down the starters, then holy shit what the fuck are they doing at their job. It's been almost a year, no way they don't have core design ideas and high-priority mons like starters and legendaries down AT LEAST in concept, if not in final design.
That said, of course we should doubt any leaks right now. Particularly because after the Decidueye/Incineroar/Primarina leak people learned that there's more credibility if you do "leak" an entire design sheet.
! There's also something about the design of that horse that just doesn't work. The other two seem fairly easy to market and work with design-wise, but the huge head on the horse looks unapproachable