We could have three kingdoms all at peace. The kingdom with the main royals, the kingdom with the main villains, and the neutral kingdom. Let's call them the Blue, Red and Green kingdoms for now. One day, the Red kingdom decides to spontaneously attack the Blue kingdom. This attack is spearheaded by the Red prince, who is a former friend of our main royals. The Blue royals are forced to flee with the Blue knight (this world's Seth/Frederick/Gunter) and take refuge in an old temple. When there, they find a Green mage who has been guarding for all his life - though he has no idea why - several magical orbs. The temple is attacked, and the Blue royals find that through touching the orbs a Hero's spirit is awakened. With the help of a few heroes from the past (one for each RPer, preferably with one previous MC and the others being regular soldiers), the Blue royals, Blue knight and Green mage fend off the attackers. Knowing the Blue kingdom's army has fallen, the Blue royals decide to find more orbs and enlist heroes into their army that way. However, the Red kingdom ends up using orbs as well, causing old heroes to appear on both sides - they can be convinced to change sides with the right people.
Does that sound like a good opening plot? The summed-up idea is to have the Blue kingdom going around the kingdoms, collecting orbs to summon heroes, defeating the Red kingdom and recruiting Red heroes to their side.