I dunno if it's what you and 3Peso mean, but one thing I've noticed on TL1 (and which I started to notice here in TL2 too, now that levels are getting high) is that speed and delays become progressively less important.
3peso didn't go into detail, but I believe that everything you said here is exactly what he had in mind in regards to balancing SPD. Like, I'm only low level 700 (with 800 library) atm, but Aya and Sanae are acting at the same time with a 9K SPD gap. It's only going to get more silly as the levels increase.
I'm going over what I want to bring up with 3peso, presently I've got, ordered as I think of them
-Fixing the way healing interacts with damage reducing skills. This is the problem with how damage reducing skills like Aya's Ability to Manipulate Wind or Quartz Charm's -20% damage reduction effect also reduce healing. In particular, Reimu's Exorcising Border + Armored Yin-Yang Orb, and Rumia's Demarcation + Realm of Eternal Darkness are instances where the character's own skill reduces their healing power. This does not affect % based heals such as Rinnosuke's Awakening spell card Precise Experience, only ones that scale off the user's ATK and/or MAG.
-Similarly, fixing how skills that boost elemental damage, such as Froggy Power and Blazing, boost the enemy's healing power, like with Photosynthesis and Regeneration.
-Akyuu's Awakening spell card Miare's Great Knowledge presently also has the effect of Protecting Art of Wisdom. Although this is likely a bug, I think it's very useful this way, and would like it to be made an intended effect of the spell card.
-Generally speaking, in the current state of Plus Disk, player DEF and MND has a small effect on reducing damage for most attacks against bosses. Because raising HP and affinities has a strong effect in increasing durability, this does not appear to cause any serious balance problems, but I would like to bring it to attention anyway due to the effects it has on character balance.
-Regarding the effectiveness of Keine's History Accumulation, I believe that the bonuses take too long to accumulate. In particular, for using Keine as an attacker, Sword is too weak to be worth accumulating. I would like to see the effect of the Awakening skill Ability to Create History adjusted so that it strengthens the effect of the Sword and Mirror counters.
-Regarding the effectiveness of Keine's skill Impervious Formation Change, by the time that it is acquired, ATB accumulation is already close to 700+ per tick, making the skill only slightly superior to Effective Formation Change. I would like to see some kind of improvement to the skill, such as giving improved delay to Keine after switching, an HP/MP recovery effect, or a stat buff effect.
-I would like to see Keine's spell cards Ancient History -Old History- and New History -Next History- have their maximum levels increased from 5 to 9.
-Regarding the subclass Dragon God's Power, it appears that its Elemental Ring passive skills are bugged and are nonfunctional.
-The boss Shadow Hata no Kokoro does not have a sprite.
-Koishi does not get Proof of Kinship from her Awakening.
-Iku's offensive abilities are somewhat lesser than that of other characters. I think raising her offensive spell card levels to 7 is sufficient. Another idea is to adjust the Awakening skill Orb of the Five Clawed Dragon to buff Iku's personal spell cards, but the loss of the MAG boost would reduce Iku's ability to use subclass spells like Start of Heavenly Demise.
-Regarding skills like Iku's Hisouten Guard and Mokou's With Keine, which boost DEF and MND, the effect of the skills tends to be low. I would like to see some kind of adjustment to make the skills more effective.
-The skills Maintenance and Prince Shotoku's Out-of-Place Artifact appear to be a little too strong. Perhaps they should be nerfed?
-When Mokou is defeated and her max HP is reduced to 1 by HP reduction effects, Resurrection puts her at 0 HP, which causes her to activate Resurrection again every time a character acts in battle (friend and foe) until she runs out of TP. I believe this is not how the skill should work, so I would like to see Resurrection set Mokou to a minimum of 1 HP.
-Miko's spell card Wishful Soul of Desire is very weak. I would like to see its healing power buffed.
-I would like to see some of the lower HP characters receive HP buffs. When facing powerful VOI attacks, especially ones following Space-Time Warp, it can be difficult for them to survive even if specifically set up to try to. This is especially true for Kaguya, who is weak to VOI. As the only stats to defend against VOI are HP and DEF/MND, and DEF/MND generally has a low effect in Plus Disk, HP becomes almost the sole defense against VOI. Alternatively, giving low HP characters some VOI resistance would mitigate this.
-Regarding Tenshi's skill Kusanagi's Former Owner, I would like to see the requirement of having a debuff or status removed. The Healer subclass skill Emergency Treatment is a 4% HP recovery effect per turn, so it does not seem to make sense that Tenshi's Kusanagi's Former Owner, which is 5% HP recovery per turn, has such a strict requirement.
-I would like to see Suwako's skill Earth Creation buffed. Presently, it is a 36% damage increase if Suwako is in the leftmost slot. Kanako's skill Sky Creation is a 32% damage increase if she is in the rightmost slot. The increased danger of the leftmost slot, and Suwako's general frailty, make it seem very strange that Earth Creation is only a 4% higher damage increase than Kanako's Sky Creation.
-Regarding the combination of Sanae's skills Youkai Buster, Power of the Living God, and Awakening skill Super Youkai Buster, I believe the overall damage increase is a little too high for Youkai bosses that are weak to SPI. There are many bosses like this, so it has a strong effect when combined with powerful SPI attacks from other characters. This may warrant a nerf.
-I would like to see Reisen's spell card Gas-Woven Orb receive a buffed PSN effect and lowered delay. It currently is not very useful, but I think that by buffing its PSN duration, and reducing the delay, it can be useful for PSN and activating Final Blow at the same time.
-Regarding Mystia's skill Soothing Type?, I would like to see it changed to have multiple levels, and restore 1 MP per level. As it currently is, it does not restore enough MP to be very useful. If it were to restore, say, 3 MP, it would make it more useful than the Magician subclass' spell card Magic Transfer, but allows Mystia to use another subclass and fulfill multiple roles.
-This is more "Because it would be interesting" than "Because it would be balanced", but I would like to see Rumia's Awakening skill All-covering Robe of Darkness buff her spell card Demarcation in some way.
-I would like to see the subclass spell card Iai Slash buffed. It is really weak even for a subclass spell card. Samidare Slash could use a small damage buff as well.
-I would like to see the Archmage's subclass spell cards Southern Cross and Execution buffed. The damage is too low for boss fights, and the MP cost is too high for random encounters. I would like for the damage to be increased, the MP cost to be reduced, or both.
-In general, ATK based subclass spell cards seem to be much more versatile and useful than MAG based subclass spell cards, for offensive purposes. For example, ATK has the subclass spells Explosive Flame Sword, Puncturing Thrust, Iron Mountain Charge, and Assassination Sword, and for characters with high ATK and DEF, Shield Bash can be strong too. But for MAG, only Aspiration Surge is strong for bosses, and the rest are debilitating effects or are unsuitable for bosses. This creates a bit of an imbalance in that characters that can use ATK spell cards effectively have more ways to strike a boss' weakness than characters that primarily use MAG spell cards. If Penetrator, Southern Cross, and Execution were more suitable for boss fights, then this issue could be alleviated. I believe that this is intentional though.
-I would like to see the Ninja subclass skills Lacquered Stones and Roughneck's Wisdom buffed. As they currently are, they are weaker than Transcendent's Enhanced Combat skill, despite the requirement of the target being afflicted by a status effect. Because the conditions for activation are more strict, I believe the effect should be stronger.
-I would like to see the subclasses Swordmaster and Archmage buffed. They are generally inferior to Warrior and Sorcerer and have little reason to be used. Their stats, while higher, do not compensate for the loss of the skills, and their unique subclass spells are not very useful outside of Swordmaster's Moon Shadow Flash. By buffing these subclasses, they can be brought up to a level of usefulness more comparable to Warrior and Sorcerer.
That's about everything I can think of that I feel confident on. Thought about bringing up buffing Shou's Bishamonten's Rage (increase the effect of each counter and lower the cap) and adjusting Nazrin's Wits of the Tiny Clever General (reduce the effect, but make it consistent instead of random), but I don't feel familiar enough with them to confidently say "Yes, this is how to make the character more balanced and appealing to use".