Here's a plot...

kinda cliche at the start

main character is a new recruit into a force that defends a city from
monsters.
he enters a fighting championchip (i.e. LoD) looses, then the king
is killed by the champion.
the queen is the one that arranged his death, cuz she wanted power.
the bad guy's motivation is that he wants the gem that the king holds
in order to access the Tower of Omen, to bridge the gap between the
current plane, and a much darker one.

my question is, what do you think his motivation for bridging the gap
to the 'dark world' should be

(also, yes it's pretty generic, I'm just looking for a short story
to practice RPGMXP)

Um...
How about a friend/family member died and the only way to get them back is to open the bridge to the dark plane. But if the portal opens, evil flows through the land and dark entities possess the bad guy's body, and uses him as a host. The only way to close the portal is to kill the 'bad guy,' and the parasitic soul within...

Easy story line to base a game off!

sounds good, I'll roll with that.

should come out good for a short story.

Good Luck!

I don't like the parasite soul idea. It's too blatantly good vs. evil. He should be an ordinary person willing to do anything to see his loved one again. To everyone it may seem like he is evil, since he would throw the world into ruin for his goals, but he's really just tormented by his loss.

Maybe she loved the evil prince

AND THEY LIVED HAPPILY EVAH AFTAH.

well I do agree that the parasite thing was kinda too g&e, but I love the relative thing so I may just simplify it to that.
you defeat him, and then you must destroy a beast that is holding open the portal?

Go with that then.
The parasite thing was just to make it sound good, don't worry, it's not my favourite idea too!

alright, I'll have a demo of it up later on today.