Okay, just came across this one...
Someone introduced me a particle physics themed 2048 clone a couple weeks back. I was disappointed, since the addition rules didn't seem to have anything to do with physics at all... they just replaced numbers with particles. (electron + electron = electron neutrino.... riiiiight) Not even increasing by mass or anything.
Well, today I heard about one that gets it right! (Or at least tries to, I'm not a nuclear fusion expert) Fe
You're trying to form hydrogen (like in stars) into iron. And the addition rules have to do with how nature really works. Nucleii decay into others, some are dead-ends... you can add them in various ways. Much more complex. But, I'm delighted to say, still fun. Maybe different fun than with 2048 -- since you do need to consult the rule-list pretty frequently. But all in all, quite rad. And I learned a thing or two.
Do you have what it takes to make iron?