It is NOT balanced in any way and I think it was part of the game design, the mechanics, the missions, and the characters do not help, it's like embrace the new and forget the old.
The balance will be difficult because the mechanics and the different character tiers, but I think at least we can start on doing more enjoyable the certain stuff.
Talking about character changes first we need to have in mind that we have two main categories free and premium/CS only and then we have different tiers based on the advantage it can give us. The modifications on characters should be done for the ones that are in day/night disadvantage against higher tiers. For example at this moment, we have tiers SS,S,A,B,C and we should work on tiers B & C to improve them to be a better tier to be enjoyable or have a chance in certain situations. Also, I think SS tier should be on limited time.
In my opinion, we should close the gap between the different tiers, having in mind that free and premium should not be mixed.
How can be done the character changes? I know there are pros/cons in the following suggestions:
1. Well the most obvious is modding the character stats/stances one by one. Which ones first? that's another topic.
2. Give permanent or a rotation buff to very old/classic chars and adjust where needed. I know there is already something similar but be realistic
3. Introduce a test server or test missions where only certain characters are modded/buffed.
4. Review the old debuff/trigger skills because some of them are not working 100% as stated as we can see against the actual stances.
Now, talking about missions and raids as said before, the new ones will take over the actual ones, and the very old will be forgotten or erased.
I think the problem with the missions and raids is the adaptability, as most of the RPG games is, so based in what I've seen in other games we can do the following:
1. Test with missions a debuff mode on the team where the debuff is proportional to the character level and/or equipped gear.
2. In a similar way as the previous point, the Boss and mobs can acquire additional buffs/debuffs/cooldown/stats based on the squad team or equipped gears.
3. Give Debuff/Buffs to the squad if certain characters are present in the mission/raid.
I know any changes require time and effort, and some changes have more impact than others, and even some changes could be impossible due to the game design.
Regards.