The type triangles in Pokemon are: Fire/Water/Grass, Rock/Fighting/Flying, Fire/Rock/Steel and Grass/Poison/Ground. You could try any of those for your game.
If you wanted a type 'square' (i.e. four starters), you could have Fire/Bug/Grass/Water, Fire/Grass/Ground/Rock or Rock/Flying/Grass/Ground.
Source: dragonflycave (
https://www.dragonflycave.com/fun-facts)
For a game with four starters to make it different, the best choice would probably be Rock/Flying/Grass/Ground, because out of all of the type squares it has the least types from the original Fire/Grass/Water triangle. The problem with choosing new starters without fakemon is that you need to make sure they're all similar, they should all have the same number of evolutions, evolve at roughly the same levels and have similar power levels. Obviously in your game you could just change things like this if you wanted to.
After thinking about it, I'd probably go for something like this:
Roggenrola > Starly > Bounsweet > Sandile
Not a perfect list, I could probably come up with something better if I tried harder, but it gives you an idea at least. You'll probably be changing trade evolutions (i.e. Boldore to Gigalith) to levelups in your game anyway, but you still might want to tweak the levels a bit anyway and maybe change Steenee to not require Stomp to evolve. I tried to keep everything single-type so as to not mess with the square, but there are no 3-stage pure Ground types, so I went with Sandile. If you're not including every generation, you'll also obviously have to change some Pokemon. If you really want to, you could always just change Bounsweet to any standard Grass starter.
