The boss design you're proud of / you had in mind

RM2K3: Shree
Giant corrupted monster tree that cannot be damaged by attacks or skills. Instead, it'll continuously summon regular enemies that, when killed, remove a portion of its HP. For every minion alive, it'll restore a little HP each turn, so the pressure is on to play aggressively.

RM2K3: Savancaya
Giant lava serpent in the middle of a volcano. Summons a wave of lava slimes each turn. Lava slimes have only 1HP, but restore HP to the boss if kept alive. The more of them that are alive, the sooner the boss activates its shield. The shield protects the boss from either physical or magical attacks for 1 turn. More slimes alive also boosts the boss's heat wave, a passive field condition that deals damage to the party. The boss occasionally also drops a meteor on a party member for massive damage
Basically, the plan is to use 1 character to fuel the other 2 characters that are capable of casting AoE magic with MP, while the 4th heals the party. Getting into this rotation is the most effective method of beating it.

RM2K3: Firaea
The volcano goddess. Occasionally charms one of the party members (he happens to be in love with her...) Every 25% HP she loses, she drops meteors on the entire party, dealing massive damage to all. At 50% HP, she transforms to a fiery state that becomes more powerful.
Basically, get punished for trying to rush her down. Recover the party after a meteor shower before continuing attacking her.

RMMV: Geolem
Giant ancient golem. Is asleep for the first three turns of the battle, allowing the party to set up boosts or take off a portion of its HP early on. Once awakened, it'll restore some HP and begin attacking. Its regular attacks are all followed up by an aftershock, dealing damage to all party members. Occasionally will charge an attack that deals lethal damage to anybody it hits on the following turn.
Best tools against him are attack debuffs, party heals and stuns/slows to interrupt his charged attacks.

RMMV: Engardien
Goddess of the Elements. Is surrounded by the spirits of the 4 elements, each of them granting a significant boost to her each turn (water= heal, earth = defence+, fire = attack+, wind = avoid+). Spirits die from one attack and grant a boost for 2 turns to the entire party when killed. Each turn, she will attempt to summon fallen spirits (100% chance at no spirits, lowered chances the more she has under control).
At the start of battle, and for every 25% HP she loses, she insta-kills one random party member (plus, she also summons all 4 spirits at once during this moment). However, the less spirits she controls, the larger the chance fallen party members will be brought back to life with full HP.
The plan is to kill off all spirits at once with AoE when she controls all 4 of them, and then have one guy take out a spirit each turn while the others whittle down her HP.

RMMV: Glaiviel
Prince of Darkness. A very straight-forward boss that does little more than attack a bunch of times in a row each turn. His attacks deal very low damage, but inflict permanent stacking defence-down, plus burn with every strike. The longer this fight drags on, the more damage his attacks start doing. Basically, take him out before he becomes a murder machine to your party. Basic tank and spank works well in this fight (though your tank will hate you for it...)

RM2K: Mecha Cyborg Jigg
An on-map battle against a screen-sized mecha Jigglypuff, lol. It has 3 points on its body that can be damaged, starting with the forehead.
Stage 1: Low continuous volley of missiles flying at you. Its eyes and mouth take turns in firing massive laser beams. It will create an orb of energy that follows you around and damages if it touches you. Destroy the forehead point to proceed and get rid of the energy orb.
Stage 2: Medium continuous volley of missiles flying at you. The lasers from its eye and mouth are larger and fire more frequently now. Destroy the mouth point to proceed and take out of the mouth laser.
Stage 3: High continuous volley of missiles flying at you. Only fires the massive laser beam from its eyes now. Destroy the eye point to finish it off.

Some boss ideas I've not gotten around to making:
Thieme:
He would fight using an atb gauge, while the party is stuck in turn-based. Basically, he gets to attack while you're thinking of what skills to use, in a game that normally only has turn-based combat. He's known for being a speed freak, as well as being a dirty fighter, so it fits his personality.

King of Darkness:
The final boss. It would have a phase in which the screen would slowly turn darker, until it eventually becomes completely black, at which point the battle would move into the next phase. Complete turns faster to make effective use of this easier phase of the fight.