Well, Mirages definitely have some positive and negative aspects. First really positive aspect that comes to mind is the fact that, as
@MZarur said, it would allow some lv70+ people who are stuck to get past that barrier and reach lv75, which is arguably one of the most important levels of the game.
The server's a bit over 5 months old and I think it's ready for more than several people to start preparing for Rebirth and hitting that sweet lv75, but the question is, is it ready for a number of people to potentially hit lv80+? Don't know.
It will definitely replace Fortune for lv70-73 (Which is what most people opt for, as Summer generally requires great gear or a decent party.) and Summer will probably no longer look like an attractive option either. Not sure how fine you are with making Mirage legitimately the best and probably the only used spot for 70+.
A few more concerns that I have in mind are how Thundoria has always been the most popular Mirage and I doubt that will cease here. The others are simply not as good, though who knows, maybe a vanilla server would push SSs to go Icicle Mirage, Cruz and Champs to Thundoria Mirage and Magics to Shaitan Mirage, as it was intended.
Next thing are the bosses that you mentioned. The EXP they give is far from phenomenal nor are the drops amazing. 50% for Illusion Heart which gives a Ring worth around 200k shouldn't be a concern, but there are the Eye/Heart/Hair of Death items which bosses drop. They are a necessary item for making Death Equipment (If the server lives long enough for Abba 5+), and that may lead to another form of monopoly, as people would generally try to stack on those items up in preparation for Death eq.
I think most of these ^ problems can be solved if the times for when Mirages would occur are thought out carefully. I like the format for CA times and I think a similar system could be used for Mirages.
Two more concerns which would probably need fixing are the entrance fees and how it would affect any exp events in the future.
Now, let's face it, the entrance fees for Mirage are a joke. I don't think upping the money cost would solve anything, but rather the Reality Mask could use some work in order to make it require
some effort, and so that entering Mirage isn't completely free. Moreover, this would make people who want to bring alts in (Buffers or tankers) walk the extra mile in order to make their leveling go extra smooth.
Now, I think that having Mirages around and all that, Exp events wouldn't really be needed for players lv70+, and should you decide to make them more scarce, the lower lvl players would suffer, or should you still put them, lvling at 70+ would turn into a joke, which is bad too. A workaround would be trying to make future EXP events and mirage times not collide, would maybe force lv70+ people to actually level elsewhere.
So, not to turn this into a bigger wall of text than it already is, I'm all up for Mirages as I think the server's ready, but there are some fundmanetal problems which may need fixing, as I said above, I think making the times cool and fair for everyone already does more than half the job.
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