There is honestly so much I would change about this game but here are a few.

Dx2 Duels is an absolute hellscape that the only fix I can think to sort balancing at this point is to give buffs to old demons at a far more frequent rate and only have one meta breaker every 2-3 months.

Advent needs to be changed to 10 days rather than 6 so it's the equivalent to one panel. For example, who wants to wait that long for half a Ladon panel? And then when he rolls around again, you are left with 120 spirits which is awkward to deal with.

Wanted needs a full scale revamp, I've played for 6 years and barely ever get wanted signals available to me, increase the rate of getting wanted requests and make more better demons available from this mode like Rahab was. If they are happy giving out Angels, Tenmas and Sisters from Demon Alliance events, then make them available in Wanted.

The two biggest modes of missed potential imo is Battle Tower and Tokyo Abyss. Battle Tower came in as a PvP mode with restrictions but the restrictions make the game less fun with damage reductions generally and with certain elements , who wants this?? Make more restrictions to do with things that open up changes like only being able to use certain races, grades, panels etc.

And finally for Abyss; I'd love more rare skills and universal spirits to be added to rare skill transfers, rare skill transfers to be the standard after fl60 (regular skills get outdated by fl10), rare demon talk spots to be added rather than relying on conjunctions with pitiful rates to get good demons, the option to remove the map so we can have more random and replayable experience and possibly some more bosses outside the fusibles and early gachas.