I want to make it clear, that I absolutely love this game. There hasn't been any one game on my phone for such a long time, and I don't see myself quitting anytime soon.
Still, I find that there are a number of problems that the game has, which I'd like to see somewhat tackled. I also would like this to be a discussion, as stated in the title. These are the problems, that I have, but if you can counter my points or add to them, go ahead. Any answers from the dev team, or signs of acknowledgement are all very much welcome.

1. The grind.
The grind in itself is not the problem. The problem comes in when the grind is not worth it. It seems getting high level materials is harder to get, or rather, we don't get them in balanced amounts and the drop rates don't seem to be balanced with the current rate of gaining characters and talismans. The soul and fragment system has made getting 4* and 5* units fairly decent and I find it just about perfect in it's current form, but recently I've found a problem. I got some Gunner characters, and I just could not evolve them to 5*. 3* evo materials are really hard to get in comparison to lower level materials, with Gunner type evo materials especially suffering from this, since the Gunner Lord of Light dungeon is practically always excluded from fragment events. The Lord of Light dungeons drops are currently awful in general anyway, which shows in the number of people who've turned away from farming the Lord of Light and isntead turned to Event Dungeons, that have a lot of great drops, even though they are still not perfect. Speaking of which:

2. Event Dungeons & Souls
The Event Dungeons are not bad, but the difficulty could be streamlined, as it seems to have major fluctuation between the different event dungeons. While the Amazing Flight dungeon was quite manageable, there have been times when madness levels became really hard to approach. But the Event Souls have many problems, starting off with the roster being quite flooded, which makes new characters hard to get. So there's the guaranteed fragments for the new and amazing and/or exclusive character to make this bearable... Except you can only get about 10-15 event souls from the events, and you'd have to buy the other souls with ironite. So my suggestion: have the dungeons drop the event souls, and slightly lower the required fragments for the character. This way it would make the grinding more reliable, fair, and rewarding, at which point the player can make the decision to either sink their ironite into grinding the dungeon, which will grant some additional rewards, or to buy the required souls to get the character guaranteed and not have to spend time grinding.

3. Pay Only Characters
I understand the company has to make money. This is simply not the right way. I'd like to get Ganglands Eddie. Ed Hunter also. Am I going to pay more than $20 (yes, more, since I'm European) for them? Absolutely not. And I think it unfair, that these characters are only available through IAPs. Even when I bought Sirius I had a bitter taste in my mouth. These characters are only available in this way, and frankly, I really didn't want to support this system. I'd suggest making them available in the future through other means.

Additionally, the problems with Evo Material drops may be solved by Lord of Light Frontier dungeons, but I can't speak from experience, since not a single key dropped as of yet, and I'm not paying FC for it. Therefore I don't feel that the new key will sufficiently help the problem, unless the drop rate gets increased.