I guess they'd have to make the center rocket stronger to handle the added force of more rockets. Also, there may be diminishing returns when it comes to adding more boosters, and at some point it makes more sense to just redesign the rocket to be bigger (hence the BFR, which is the planned successor to Falcon Heavy).

Musk has said there is no reason they can't attach two more in the future.

That would be simply Kerbal.

Honestly it’s amazing how much better I understand this and other space stuff I’ve seen in the last year or two (like shows about Apollo, etc) since playing KSP.

No better way to learn basic orbital dynamics out there.

SpaceX should create their own space simulator or buy KSP, let players test and sort out various problems - crowdsourcing! :D

They don't need to buy anything. Related visuals (models and textures of KSP rockets) were already made and are maintained by the community, and all they need to do to crowdsource a test is to post it on Reddit. I guarantee you, hundreds of players will try to one-up one another on that one.

(Hell, I might even be among them.)

The fairing diameter would not be much larger than it is right now and that is a limit all by itself. A bigger rocket would support a larger fairing.