I just finished my first playthrough with this mod; here's some feedback.
Potion Gems/Crystals/Stones/Relics/Artifact were really nice; having to use an accessory slot, but being able to combine them later on, made for a reasonable progression.
That said, I was playing a mage, and the Mana Regeneration Gem does not work as advertised. The Mana Regeneration Potion essentially modifies the mana regen formula to always treat the player as standing still and having max mana, as well as reducing the delay before regeneration starts. I don't know exactly what the Gem does, but it is still dependent on current mana, and doesn't reduce the initial delay nearly as much. Is this for balance reasons, or is it a bug? The wiki goes into detail on how the potion works here:
https://terraria.wiki.gg/Mana, which is probably a better explanation.
The various normal-world bosses were fun fights, with the exception of the Steam Train, though I think that's more a matter of my arena and the Electrospark being OP.
I did encounter a bug with Dioritus/Andesia/AnDio though. My first attempt at fighting them I died to Dioritus, and my second attempt to Andesia. But on my third attempt I fought (and beat) Dioritus, Andesia, Andesia again, Dioritus and Andesia together, AnDio; I assume that the second Andesia is not supposed to be there. Also, on subsequent fights, the battle is only Dioritus and Andesia together, AnDio; I do not fight them individually.
The weapons I tried (all the magic ones) seemed mostly balanced, with the exception of Electrospark. Being able to pierce through blocks and that high a number of enemies with high damage, fire rate, and spread is too much. I'd suggest removing the piercing (blocks and enemies), since the Steaminator fills that niche, and it would give more incentive to space the projectile so it has room to split.
The Etherial is a cool idea, but needs work (which you've noted in the OP). The Etherial versions of most earlier bosses don't hold up to post-Golem firepower, and the Golem itself is one of the easier bosses to start with. Plantera and the Steam Trio were the only ones to put up a decent fight, though Skeletron had a nasty surprise. The Etherial items also range all over the place in regards to balance and usefulness. Here's my thoughts on them, in approximately boss order.
- Etherial Gel: It's not bad, but there are plenty of other accessories that do this or better, especially wings.
- Eye of Etheria: I assume this is going to be more relevant as the mod develops; currently I don't see much use for looking across the dimensions.
- Ascension: This one was good; it's hard to go wrong with better wings.
- Etherial Scarf: Bland, but mostly balanced.
- Fragmented Mind: Potentially a good risk/reward item, but I rarely find myself in a situation where I can afford to take a few hits, unless my Defense is so high I can just tank things outright. This may just be what I get for playing a mage though.
- Byfrost: It seems a bit weak, since damage over time effects only really work well against bosses.
- Etherial Pack: The effect is basically useless; honey is very rare, meaning this only works if you build an arena specifically to make use of this accessory.
- Molten Etheria: Lava is more common, but this is even harder to use in arenas/most boss fights. The max Life bonus does at least give this some general-purpose use.
- Etherial Skull: Hilariously overpowered; I've managed to get nearly a 600% boost in damage from this accessory.
- The World of Etheria: I haven't used Time Stop yet, so no comment.
- Etherial Energy: Another accessory slot is always welcome.
- The Destruction: On its own, this would probably still be too powerful. With the Etherial Skull, however, you can easily tank even the Moon Lord and get a massive damage boost from it.
- Mechanical Rage: I wasn't able to properly test this one, but the basic idea seems good.
- Justice: Potentially very good, potentially worthless. It all depends on whether that one hit was a big one or a small one. Overall, I think it's fine, though it's not my cup of tea.
- The Annihilation: "On kill" effects have the problem of not being good against most bosses, which is where you most want big damage boosts. It's not bad, so much as it doesn't have a good time to use it.
- Etherworks: This, on the other hand, is perfect for events. One enemy dies, explodes into cogs, and potentially kills another enemy in a chain reaction. Having the debuff means it still has some uses against bosses too. It's probably my favorite of these: a relatively balanced unique effect.
- Etherial Tank: The sound on this gets old quickly; maybe tie it to the visibility toggle? I couldn't get the collision part to work right, and the formula is hard to understand without knowing what value is used for movement speed. It's hopefully not mph, because I was up to 100 mph on Asphalt, which would give 50,000 damage.
- Etherial Sac: This one feels okay. Being grappled to a block is a dangerous thing in the endgame fights, but this provides a high payoff, and the life regen helps mitigate the risk. It also has synergy with the Antigravity Hook
- Stone of Etheria: This one can get out of hand with the Etherial Skull/The Destruction combo, but will probably be fine once that gets fixed.
- I couldn't find an Etherial Duke Fishron summon.
- Etherial Globe: Being able to take your shiny new toys anywhere is a good payoff for beating the Moon Lord.
The Bysmal armor could probably use a better explanation for how it works. I'd also appreciate a way to "lock" the effects; currently I've got Etherial Skull/The Destruction/Etherial Globe for effective godmode, but fighting any Etherial boss would mean fighting Skeletron/The Destroyer/Moon Lord again just to get back to where I am now. Also, this really needs something for summoners, because they don't get any minions from this armor set.
The Bysmal tier is also missing a magic weapon, though it's not really a big deal.
I liked the sprites, though something about Slybertron seems a bit off. I also liked the music, especially Obsidium, Ragnar, and Etheria. And this is more of a personal thing, but I appreciate that the mod isn't constantly flooding my inventory with items I have no use for-- that's one of my biggest gripes with a lot of content mods (especially Thorium). Overall, I plan to keep this mod, and I'll probably give Mystic a try soon.