They could give the new starters more useful abilities. But that seems a bit unlikely.
Water's usually a good defensive type. So there's the possibility we could see a new and useful bulky water. The role is crowded though.
Fire tends to be more offensive. Infernape does such a great job there though. Of course it upstaged Blaziken, so it could in turn get upstaged itself.
Grass is generally mediocre. But maybe give the starter a great movepool - strong attacks and a buttload of support moves.
In all three cases, for the new starters to stand out I reckon they'll need either unique abilities (unlikely), good signature moves (possible, but knowing Game Freak a rubbish signature move is just as if not more likely), or useful secondary types (most likely).
Not sure how likely it is, but Grass/Electric doesn't look like a bad typing. Defensively grass loses its flying weakness, while electric loses its ground one. Still 4 weaknesses left mind (Poison, Fire, Bug, Ice). Offensively grass hits ground (which blocks electric outright), while electric hits flying; you also hit every water type bar Kingdra, Palkia, and Ludicolo for SE with your STAB, and you get a fair number of the dragons neutrally. Opposing grass types wall you though.
Of course, the starters might not be grass-water-fire.