Having played for a while now and being firmly into the farming monoliths part of the game I thin there are some quick tweaks you could make to spice up the different monoliths and make the runs a little more dymanic.
The easiest ones are the spire ones, you have the different types but they all effectivly do the same thing. Big AOE telegraphed well in advance.
Some quick thoughts on how things could be changed are that each type should attack differently
Bone - 2 types of attacks, 1 a circular bone prison that traps you until you smash your way out, or a string of bone shards that land in a line (Instead of the current circle)
Blood - Where it lands a pool of flesh lands, this morphs into a creature that attacks. you could easily have 3-4 different ones.
Moon - Any mob that is in the circle gets boosted up to the next rarity, this means you could use it to farm harder mobs adding more strategy. It could also apply a debuff to you.
You could easily add in 4-5 more that all have different effects, fire walls, different buffs to mobs, a tracking laser beam that chases you the whole level until you take it out, time rewind to res a bunch of stuff etc. Some of these I imagine would be really quick to implement and would greatly improve the overall variety.
I would also drop the ambush ones and just make ambushes happen in every 4-10 echos that you do. Once again, adds a level of randomness and makes the echo more dynamic, Plenty of mini boss level mobs you can throw in for ambushes.
Please add wave number to arena style echos, like you have in the arena.
For the arena style ones, maybe add a mechanic where you can summon the next wave early, but provides a boost to the loot that drops off the last monster killed. Much like in tower defense games, the faster you call them the harder it is but also the better the reward.