This is the initial idea, I'm probably going to implement them in stages. I know it will be a long mod to make, but you got to be ambitious to stand out
I'm currently doing a Games Design course at uni, so this should fit in quite nicely with it (mostly scripting and designing).
I think a good target at first would be to get the similar races into the game, Empire: Humans, Chaos: Krax, Elves: Wraiths. A lot of them are re-skinning jobs.
Just made a stage one plan, I want to get a few races down first, re-skinning will be needed, but nothing too hard 
Next will probably be getting the equipment done.
I'm thinking the best way to go about this is in batches of races. Do a few races at a time, do there equipment, then the magic. I think I'll leave magic until last, but that may change, as magic is quite core to the warhammer universe
As for the game world, I'm going to sacrifice story for playability. It will probably something along the lines of extending the warhammer story so that it is after some kind of cataclysm, where the races are starting out again.
As for the units I will try and make the set units, but still give the player the option of creating new ones, as that was one of the things warhammer was about, customizing your units with different weapons, armour and items.
For research, thats going to have to be a whole new tech tree I think, and for now I'll try and incorporate it into the current one, with a few modifications.
So here's my reduced version proposal:
Only do 4 races, using current races, re-skinned:
Empire(Human)
High Elves(Wraiths)
Chaos(Krax)
Orcs(Troggs or Urxen)
Once the races are done, I can work on expanding them with there own equipment.
Oh and, I'm going to try my absolute hardest to keep this project running 
EDIT: Just had a go at making elves from Wraiths -

Now to mess with the settings 