Personally, I think the whole "human turning into a Pokemon" thing is cliche at this point, especially since we never see any other humans or any implications they exist other than the player's existence (I heard theories of the PMD world being a post-apocalyptic world before). Though the only reason they do that is most likely because they want the player to relate to the protagonist more because the player is human him/herself
At least they're bringing back the personality test, which is a plus, even with the limited typing in the starter selection.
While being able to play as all the starters is a blessing, you are right in that it gives you very limited options for types. The main-game starters are always portrayed as the protagonist's ace Pokemon in the main games (hence making them a protagonist in their own way), so it's natural they have them dominating the roster. Riolu's presence was mainly due to the popularity of Lucario, and Pikachu is too obvious to even explain.
And for clarification, Pikachu has been a main-game starter. It's the one you get as your partner in Yellow, and acts basically like Ash's Pikachu from the anime.
I kinda liked there was more variety in the earlier PMD games, but as we got more and more new main-game starters as the generations went they put more focus into those (as we know, starters always get tons of marketing and are generally popular because they are your first partner after all) and sadly it came at the expense of those other Pokemon. The issue is that they just love to favor the most popular stuff because they appeal the most to the targeted demographic who follows the franchise.
I played Yellow before so I already knew that Pikachu was a starter, though I wouldn't say the starter is the main series protagonist's ace Pokemon, especially since the entire point of team building in the main games (be it competitive or not) is to essentially expand from the starter, not limit yourself to the starter. I mean, the first game had Cubone, Meowth, Machop, Psyduck, Skitty and Eevee as starters, as well as Pikachu and the starters, and EoS had the starters, Pikachu, Meowth, Munchlax, and also added Riolu, Vulpix, Shinx, and Phampy, and also re-added Eevee.
It would make more sense if they had a Gen 6 Pokemon (asides from the starters) to further clarify this as a Gen 6 game, and I think something like Pancham, Espurr, or Goomy would make more sense than Riolu, a Pokemon from Gen 4. The main problem with the starter selection is that, like I said earlier, there's no variety in typing, especially since the story refuses to evolve the player Pokemon until after the main story is over (and 90% of all starter Pokemon don't have secondary types until their second or final form), not to mention how the main story always discourages the use of recruited Pokemon from dungeons, so there's really only 5 Pokemon you can play as when you really narrow the selection down. I know Lucario is very popular, but there are numerous other Pokemon they can use.
