Probably for the first time ever, I'm going to say I don't want to do a direct clone. Even without considering the collaboration I was wanting to do something with a very similar concept… and make it for resale. (Or at least get it up in some stores for a free download). I'm not too concerned (or even expectant) of actually having a complete game ready to go to market as a result of the collab… anything that would move us in the general direction of having a working game will be great. I can always use the experience (and possibly reuse some code) to build on later.
With that being said, I think there would have to be a few changes just to keep everything on the up and up so to speak, for trying to resale the game. For instance, all new art work. A little theme modification (not word cookies perse, maybe letter monsters or alphabet soup, etc). An interesting thing about the word cookies, is it has several different themes. There's a fall (Halloween), Christmas, Ocean, and a couple other things which are neat but little more than a graphics set change out. For our collab, thats a bit much to try and get done, but I think it's useful to keep in mind to avoid artistic duplication as much as possible.
I would suggest giving it a download and playing it a bit, if for nothing else, it's a fun puzzle game and will give everyone a decent idea of what we would be shooting for. Also, I'm good with MonoGame. I'm still not super experienced with it, but not brand spanking new either so I'm sure we could make it work. Unity on the other hand is basically something I have never touched.
As a last note, I would like to do a little high level planning before we get started. I think having some kind of basic layout is far superior than going in blind. Although we've had pretty good success with both.