Greetings. I've been enjoying this game a lot since I started playing so I figured I'll give my take on this topic. My biggest suprise in the store was the lack of a crystal bundle of some sort. I've seen a few topic that there was one in fact, but as a daily purchase.
Before trying this game, I played another that had a incredible monthly crystal pack so I figured I might as well suggest it here. The bundle was about 15$, and gave you about 3$ worth of crystals every day, for a month. There was also a VIP system but that is unique to that game and I doubt it can be easily reproduced here, since the highest VIP rank required about 15000$ put into the game.
The first question I asked myself was "why would you make a pack worth 15$ and give out 100$ worth of crystals when you can sell 100$ worth of crystals" and after working on a market similar to microtransactions it was quite clear that this bundle will bring a lot more profits. On my personal example, I was without a credit card to my name and no means of paying online (quite common in 3rd world countries) but when I saw this crystal bundle, on my visit to the USA I bought a 100$ google store card so I could buy these packs for a few months. That is a 100$ from a f2p player. In the long run, many f2p players buying cheap packs brings in more profits than a few whales buying big packs.
Of course, not every f2p player will buy this. But a lot of f2p will go p2p given a "great bargain" as they would see it. If the system was not good, there would not be so many mobile games using it.
Of course, just a suggestion. I saw the devs were looking into other means of bring crystal packs back so I figured I might as well throw this in as a very good option. Feedback is always appreciated.