Personally I'd have a sentient nanomachine-cloud from seemingly nowhere descend upon Earth and talk to someone (e.g. Clark or Lois) as a hologram of a childhood imaginary friend, claiming it's bored and challenging Superman to play its game for reasons unknown.
And I'd try to get Peter Dinklage to be that hologram.
EDIT: And in the end we'd be left like "So did Brainiac 5 send it from the future to prepare Superman? Did New Genesis or Metron send it? Is it just from the fifth dimension like it claims?"
Also, this
:
http://www.imdb.com/board/bd0000130/nest/263626706

they'd have to find the right actor to do the voice. i don't think it could be done live action, it would have to be CG
IMDB taught me: cynicism is praised. optimism must be stamped out wherever it can be found.

Mister Mxyzptlk could be in Supergirl or something, if they really felt like doing that. I doubt it would work in the movies though.

no Mister Mxyzptlk is a bit too difficult to translate into live action. though it would be interesting to see if anybody could top the performance of Howie Mandel.
IMDB taught me: cynicism is praised. optimism must be stamped out wherever it can be found.

Especially after Johnathon gave up his life for Krypto in that Man of Steel movie
My bestestest funny evah!!!
https://vid.me/ToVD

No the only things that could have saved this movie is Ace and Krypto silly
To make a great film you need three things - the script, the script and the script -Alfred Hitchcock

No, because then it will not all end with a bang or a whimper but with this:
http://static.comicvine.com/uploads/original/0/40/2045118-524293_ew___superman_batman___world_s_funnest___36_super.jpeg
.
Also, this
:
http://www.imdb.com/board/bd0000130/nest/263626706