This is just my suggestion:
I think the next shape is a circle again.
If we associate these shapes with the number π (3.14159...)
A triangle has 3 ends - represents 3
A circle can be taken as 1 (wholeness, no corners)
A cross has 4 ends - represents 4
The next number in 3.14159... is 1, and the circle was previously used to represent 1, so the next shape is a circle again.
Following the logic of π (3.14159...), the next number is 5, so most likely, after the circle comes a five-pointed star.
I don't know which shape to choose for the number 9 🤭😅