Most basic usable version. USB powered, uncentered bubble, not size constrained, no pressure sensor. A setup to test programming. Slapping parts together that were already made for other projects.
Adding pressure sensing as well as making it battery powered. Desktop bubble v2 is big, bulky but functionally complete. (maybe sans wireless connectivity added later)
Work on case design, look at power requirements and write a how-to on assembling it. The Desktop Bubble v3 has now a nice case is presentable and desirable.
Stress testing, material selection, prize optimization, Fix problems found along the way, extra features, think of actual use cases.