I had always thought of a Sinnoh 3 stage Grass Fire and Water trio for Dawn together with Buneary Mamoswine and Togekiss

Since she already has Piplup that covers the Water part. (I doubt it will ever happen but maybe in a near future Piplup can decide to want to Evolve into a Empoleon)

Fire is easy since there are not many options. 2 exact (i literally limited to Infernape and Magmortar and i rather keep it to 1 starter so Magmortar it is)

For Grass i have also 2 options (Which are Torterra and Roserade. Since i cant use Torterra for obvious reasons and Roserade is the only other Grass type with a 3 stage that made it my choice for Grass)

Dawn has a Glaiceon correct? I think a Gen 4 Eonlution would be cute but if I wanted to mix it up I would choose Eldegoss becuase I am playing a Nuzlocke right now and that plant is a beast! And it's cute!