I expect that most us reading this post have played, or continue to play, a pretty decent variety of miniature (and/or board) games. For some, a few of those games were originally purchased because the game was fun to play and had lots of promise. Unfortunately, instead of growing more fun, or even maintaining the current level of fun, the game jumped the shark, as it were, having introduced new content that ended up sacrificing the former balance the game enjoyed on an altar intended to provide innovation and expansion, but ultimately ended up diminishing the game to the point of making it nigh unplayable.
Many of those who have suffered through the demise of one or more great games would say that they have experienced first hand the old proverb, "Once bitten, twice shy". These are the ones who are especially reluctant to expand and enhance what they regard as a good, solid game. Not that they shun change, but rather that they want to avoid the sort of pitfalls that they have seen other games fall into.
The one thing that I think is more (potentially) destructive than any other thing one could do to erode and eventually destroy game balance, is too introduce too much synergy.
You know what I mean when I say synergy, right? I am talking about card combinations. Abilities which by themselves are fine, but when coupled with certain other abilities become a card-combination that is greater than the sum of its parts. There is a reason why Soontir Fel is played with PTL, because the PTL gives Soontir a free action, and since Soontir's dial is so green, the stress he receives is (for all intents and purposes) negligible. That is an example of synergy. Keyan Farlander with a PTL has a similar synergy, He can PTL for a free action TL, and spend the stress gained to change focus results into hits.
Normally PTL's positive effect (free action) is checked by its negative effect (stress token), but in the case of pilots like Soontir and Keyan, PTL goes from Postive/Negative to Positive/Positive.
That isn't to say that PTL is necessarily an "OP" card. For pilots that can turn its negative effect into a positive one, PTL provides a very effective (both in cost and action economy) synergy. In fact, the only ships you every see using PTL are those ships with the preexisting advantage of a mostly green dial. In other words, an upgrade like PTL is only going to see play in situations where it's negative effect is negligible.
Now, I am using that as an example of what I mean by synergy - the introduction of a game play element that grants unevenly distibuted advantages. Another way of saying that - is the introduction of a game play element whose situational value varies from ship to ship, but whose (point) cost remains constant regardless of usage.
I don't want to bog down this post any further discussing PTL. I love the upgrade, and use it all the time, on pretty much every fleet I field - which itself says something about how "synergy" - instead of increasing your options, it ends up producing "must play" combinations that offer too much of an advantage to sit in the drawer while you field other things.
Now having a few combinations like that doesn't necessarily unbalance the game, since there are all kinds of ways to counter or disrupt that kind of synergy. So I am not suggesting that the ability to produce synergy in the game is bad or game breaking. Good gravy, no!
What I am saying is that the amount of synergy in the game increases exponentially with each new expansion. Think that through. Each new expansion introduces new upgrades which can interact with existing upgrades, to produce new synergies. There are only a few at first, but because the increase is exponential, we can expect each new expansion to introduce a greater "burden" (if you will) to the balance of this game.
I am not saying that if FFG introduces too many upgrades it will necessarily collapse under its own weight, but I am suggesting that this has been the oft repeated pattern I have seen, and the reason isn't necessarily the expansions themselves, but rather the fact that it is increasingly difficult to police the way new combinations interact. Eventually, the number of rulings with regards to card-combinations etc. will become convoluted, and it will take longer and longer to resolve the problems that are introduced, simply because there is a lot more to test with each and every upgrade introduced.
I think that the hardest thing to balance in an expanding game such as X-Wing, is the various synergies introduced through new expansions. I don't think it is impossible to manage them - but I do think it'll take exponentially more work to manage them properly as new expansions are introduced, and I think that if FFG doesn't rise up to meet these coming challenges in a timely way, the game may well suffer the same (albeit predictable) internal collapse we've seen in other similar games (I'm looking at you Star Trek Attack Wing!)
TL;DR - I think that as more expansions are added, the risk of unbalancing the game through a multitude of synergistic card combinations increases, such that I hope FFG is already taking steps to mitigate against a trend that has sidelined other successful games who expanded too quickly.