Im going to have to go with clockwork on this one. While I do think there are a few endgame combinations that could be added, in general there should be a thematic reason or need. Beyond that, I think the current accessories are fine and don't really need to be combined any further, because, as mentioned, forcing the player to make choices is a
good thing!
I also disagree with the idea of extra accessory hearts from other bosses. There is NO sudden cheese strategy that is suddenly possible with 6 accessories instead of 5. And if you do desperately need an extra accessory slot (for some reason), you can get one for free simply by playing expert mode, and
another (bringing the total up to SEVEN) after wall of flesh. In my opinion, that is already plenty of freedom of choice, but yes, key word being
choice.
Combining expert accessories has been suggested before but there is probably a balance reason they cannot. If the accessory is already good enough, it doesn't need to be combined to be worth a slot.
I do like the idea of moles that pop out of the ground as long as they do NOT destroy blocks. Why not have them just dig the same way worms do?
I do like the idea of a hanging railcar of some sort, perhaps that deserves its own post? It, respectfully, doesn't deserve to be lumped together with these other more controversial ideas.
Some sort of gravity dart that makes the critter rise seems fine to me, and should be little issue in terms of AI. But would obviously be very strong if it also worked on enemies.
I agree with the
idea of more crafting ingredients since it would open to door to more future weapons/items. (I previously suggested both
chaos fragments for RoD, and Manipulation Fragments from the cultist which would be used to craft the ancient manipulator
and other items, instead of dropping dozens of the manipulator every time you wish to refight the moon lord). The
idea of a moon lord heart, tear, or fragment of some kind is cool, but PLUS FIVE HUNDRED health is ludicrous, and that armor is .. admittedly okay if it weren't for the "mirror vanity set bonus" part. I feel like that would be easy to cheese, such as crimtane armor (? the one with boosted regen). Those other sets with powerful set bonuses are often balanced by lower defense/stats, an issue that would be COMPLETELY negated here. If we were to add luminite armor, its best to keep it simple, such as just giving defense and that's it.
Second is the problem of needing (by my calculations) TWENTY-NINE MOON LORD KILLS for the full set and core you suggested, plus any other future items that would require it. This should, if anything, be one fragment per piece. That would mean you would have had to beat the moon lord three times for the full set, but at least its guaranteed and not reliant on drop RNG. That, actually, is a cool idea for limiting excessively powerful items. You were on the nose with the concept, but swung way too hard in the implementation.