making bosses better, how could it be done?

There are plenty of ways to make boss fights more engaging and interesting, but two problems need to be addressed first:
1) The large gap between funded and unfunded players
2) The homogenization of classes

After that, a bunch of different things could be incorporated based on classes' strengths:
Large groups of summoned mobs which heal the boss, for mobbing classes
High hp, low density of summons which give buffs to the boss, for single target classes
"Armor" stat on monsters which reduces damage per line, to encourage/discourage multi-hit, low dmg-per-line skills versus single-hit, high dmg-per-line skills
Avoidable attacks which are actually dangerous, like how Anego slap was, to encourage ranged classes to actually leverage their range rather than tank everything and pot
Make potions recover over time instead of instantly (and with short cooldowns), so that a strong DoT casted by boss can actually kill players

There's so much more Nexon can do to improve the bossing experience even though Maplestory is 2D. Use your imagination.