New theory: the DLC Will drop on Monday 22 April which is Earth Day.
The North America animal pack was released on a Monday to match World Animal Day.
So if my idea is not wrong, it would be like this:
March 26: North America and Europe (temperate packs)
April 22: new tropical-ish DLC
May 28: aquatic and twilight
1 month approx between each DLC and no overlapping of themes. For me it makes sense!