I guess you just learn at what is interesting and what not if you make enough worlds.
For this world I also took a lot of other things from other games/movies/whatever.
For example the idea of death riders came from warg riders.
The no one is technically evil came from how I look at phyrexia.
undead came from {insert random horror thing here}
I also changed the undead quite a lot, they used to be more technical however when I got around and think about there back story this got changed as I wanted to make the world before the destruction as alive as possible to make the contrast between the world before and after as big as possible.
This has the site effect that if I ever make a game out of it they are the only ones without long range weapons so for a FPS it would be interesting to play as them and make them balanced (in case of multyplayer).