[Delphiki]

Just a few ideas (keep in mind these are pretty directly inspired by the ideas in this thread, maybe slightly re-hashed and re-worded):

Two-finger drag up to zoom-in, centered on where you first start the drag. Two-finger drag down to zoom-out, centered on where you first start the drag.

Draw a circle (user can set whether clockwise or counter-clockwise) to literally outline what you want to zoom-in on. Draw a circle in the opposite direction to zoom-out, centered on the centroid of the circle.

Similarly to the reverse function in Paper by 53, maybe a two-finger circular motion to zoom-in and zoom-out. Alternatively, this could be a neat way to navigate forwards and backwards through your internet browser history.