2.022 It is obvious that even a world quite different from the actual one must have something -- a form -- in common with it.
Both Ogden and P&McG refer to an “imagined world” in contrast to the actual one. Black (p. 62) points out that this is wrong and that “a world which is thought about” would be more literal. I think the least clumsy way to render this is to leave out altogether a reference to the non-actual world’s being merely thought of.
See 2.0141 on the form of an object. The concept of form seems to have to do with possibility. What all possible worlds have in common is, precisely, possibility, i.e. form, i.e. logic.