I could see the Summoner/Spiritualist-style of Pokemon Trainer as a base-class chassis (preferably a Caster-type).Similarly I could see Legacy/Phantom Weapon Wielders and Soul-Knives/Mind-Blades/Solarions, etc as a seperate base-class chassis (preferably a Martial-type with Spell Points; an arcane/occult foil to paladins.)
I'd actually prefer if they kept the Summoner as its own distinct class, focusing on extraplanar connections and the relationship between the summoner and the eidolon, and they cast off of spell points for summon spells, rejuvenate eidolon, etc.
As for the Spiritualist, a good way to go about it would be to give them the option at first level to either A.) go the road of the summoner (Path of Possession?) and gain a fighting ghost buddy, but you're restricted to spell points, or B.) go the road of the gish (Path of Death?) and gain an ectoplasmic weapon, as well as increased spellcasting to make up for the comparatively reduced action economy.