Over the past 2 Gens, game freak has set a couple of starter trends that I think they plan to finish up with in gen 8. These trends are:
A bara/waifu/edgy trio
One of the members of the trio has the dark type, and the other members' secondary types form around it. Thus, i believe that our next round of starters will be the following:
Grass starter:
Grass/dark waifu
Slowest of the trio

Fire starter:
Fire/bug edgymon
Perhaps a worm-snake
Fastest of the trio

Water starter:
Water/Rock bara
Medium speed

Does this sound plausible?