An Easter egg (hidden surprise) that links you to another page with a secret element on it. It would only be unlocked (the secret link) by combining future elements together mixed with old elements and making a secret combination that would either directly make the element or create a pop up link to the element. Plus the page with the secret element would have no number code so it couldn't be acsessed through a link ;)
@Turban-of-Terror(View Post) @ZER02541714(View Post) That won't work, the page needs an ID to be in the server, also, anybody who is looking at the source can easily find the element and spawn it via console.
I think it is very likely that some form of movable solids will be included (as an option). I suppose you remember that water equalisation was also thought impossible to code at one point.