What I do to regrow grass is plant a lot of shrubs and trees.
If I cant place any where I want the grass to regrow, I use flowers + watering them but it takes forever...
I tried growing this line of grass back near my house and it took near 2 weeks.
I didn't step over it at all. If villagers step on the spot, it's alright but if you do, it'll negate your watering if you did.