These are Gigantamax Eeveelutions as I imagine them.
Vaporeon now has a more spread out "mane" of fins, in addition to having fins on the paws giving a winged shoe effect, and a tail that is larger at the end, resembling a folding fan.
Jolteon has a larger mane, shaped like a V, as well as white fur on the paws giving a winged shoe effect, and the fur on its back is white and spikier.
Flareon has a much larger mane, and its tail is an actual fire. Its ears curve upwards like bull horns, and the tuft of fur on its head becomes heart-shaped.
Espeon has larger ears; the inside of these, and the (longer) under-ear fur, are yellow, in addition to the tail. The gem on its forehead is now a 7-pointed sunburst shape.
Umbreon now has crescent-shaped ears, tail, and marking on forehead. The ears and tail have yellow tips at both ends and join Umbreon's head and body in the middle of the crescent. In addition, it now has a crescent-shaped mane with the middle section yellow.
Leafeon has a larger tail and leaf on forehead, and a flower on its collar. There is also a flower each at the base of the ears, the base of the forehead leaf, and the base of the tail.
Glaceon now has darker body fur, except for the light belly and hind legs. It now has two tails, both navy blue. It has a braid reminiscent of Elsa, and a snowflake each as crown piece and collar piece.
Sylveon now has a tail shaped like a music note. Its bows are replaced by blue flower-like elements, and each ribbon holds a blue maraca.