Biolabs mutant, 100m HP, all skills of all 12 Bio3/4 MVPs, form of creature changes on HP but is randomized, skills are not based on current form. Shifting stats of 1 vit (counter bomb), 300 luk (counter crit), 700 flee (counter normal hit) stat piercing debuffs so you're forced to bring multiple resistance gears, lv10 EQ with a random cast timer and that anti-token skill from SM (Hell's dignity? not sure)
I'm sure I can think of some more evil things, but I am all for something like this
an mvp like this would be difficult to be sure, but more annoying than anything else as far as taking him out. what sort of teamwork/coordination will be required for this fight? you have strings, ME, healer or two, tank, and fill the rest of the party with RKs and Genes. anyone who isn't one of these classes would be there strictly to absorb EQ dmg and spellbreak; their own skills are utterly useless. the class composition is really no different than a group you'd take to bio4, ET, or just normal BB popping. hell the strings and ME could even be dual-cliented and would be left alone until you needed to refresh the ability.
because it's probably impossible to make mvp mechanic changes like adding an agro table, I feel the easiest option to implement would be to have things happen throughout the encounter that will require people in the party to take care of. things such as:
maybe an npc shows up during the encounter that requires healing and will reward a buff to the group or help damage the boss/remove some sort of immunity
monster spawns that need to be wiped out for fight to continue
mechanisms that need to be triggered in order to prevent the mvp from casting an insta-wipe skill
you don't necessarily need to make the mvp hit super hard or have a humongous health pool (there's a kRO vid of an RG soloing the nightmare OGH MVP so it clearly isn't all that effective). by making the encounter more immersive you can have it remain challenging by making it more skill-based and not something you can just outgear. doesn't matter how shiny your equips are if you can't coordinate well enough to prevent the insta wipe ability from going off.