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Pokemon Trainer Official Master List coming through
If whichever Pokemon trainer we get follows the same format as the Gen 1 trainer we can assume the following:
>1st stage is likely to be small and fast, with 'cute appeal'. It has weak attacks but high mobility and is more likely (but not certain) to be a water-type.
>2nd stage is likely to be a middleweight quadraped, with a somewhat unorthodox fighting style that goes beyond the simple dynamics of the other two ('small but fast' and 'slow but strong'). It is more likely (but not certain) to be a grass-type.
>3rd stage is likely to be a hard-hitting heavyweight, and the 'cool appeal' of the trio. It is more likely (but not certain) to be a fire-type.
Gen II: Chikorita and Cyndaquil both have the cute factor, but Cyndaquil has more mobility thanks to Rollout. The only thing Totodile has going for it here is its water-typing. Bayleef is the obvious choice for stage 2 based on the above criteria. It's between Feraligatr and Typhlosion for the heavy-hitter spot, while Typhlosion is a fire-type Feraligatr is the more physically capable. This one is a toss-up, but assuming Cyndaquil takes stage 1, Feraligatr gets stage 3 by default.
>Cyndaquil, Bayleef, Feraligatr
Gen III: Treecko fits the high mobility profile perfectly, though it's undeniable that the other two are cuter. This is a toss-up between Mudkip for its water-typing/cute factor and Treecko's speed. None of the Hoenn mid-evos fit Ivysaur's quadrapedal template particularly well, but Marshtomp fits the weight and damage profile the best while Grovyle has the grass typing. Blaziken is the obvious choice for the 'cool-factor' final-evo and is inevitably what the devs would push for, though Swampert fits the slow, heavy-hitting template much better.
>Treecko, Marshtomp, Blaziken