I think I'd gate it as three feats.
First one, available at first level: 2/day, you can assume the form of a simple creature you've studied (e.g. domesticated animals, simple forest creatures, etc.) Limited to weaker creatures, CR does not scale, etc. This is utility shapes only, no flight, no water breathing, pretty basic stuff.
Second, available at level 4, basic combat shapes but not super great scaling, 3/day. Can do water breathing creatures, and "limited" flight--essentially songbirds and other things that can hop over a wall but can't fly around town.
Third, available at level 8, full wildshape, better combat forms, but still inferior to an actual, proper Moon Druid, PB/day. Far fewer restrictions on what you can become.
Of course, I wouldn't actually do this in general--I am opposed to turning the class system into a pure all-you-can-eat point-buy, as I think that change in design significantly distorts gameplay--but if I were to do it, something like the above is how I'd handle it.