At most, he came up with the idea at some point during the 23rd Tenkaichi Budokai, but it's almost entirely possible he decided on a whim to make Goku a space alien to keep the story moving.

Starting back in 2005 on the PlayStation 2, the first Budokai Tenkaichi game pitted players in a third person combat setup that has pretty much laid the groundwork for many other Dragon Ball games ...