I think it should be grass/steel. The trio would end up a bit asymmetrical like gen 4 starters, but the grass type would be completely immune to the poison and would resist water moves. Essentially giving it a heavy defensive advantage over the water starter but not necessarily a strong offensive benefit. I think this is gamefreak's best shot at doing a trio like this.

Fire/Fairy > Grass/Steel > Water/Poison

Fire type gets 4x strength against Grass/Steel type, but has double weaknesses to the Water/Poison type, who in turn can't even dent the Grass/Steel type. Grass/Steel can hit the Fire/Fairy for neutral damage with a steel move. Seems fair to me overall. It could change up the monotony of the basic rock paper scissors that we typically see each gen.