One could always say perhaps that there has always been a resistance to the peace, which is what makes our kind necessary, and now they have found something, or have created something that will help them to bring back the war, and hopefully, wipe out the other side. The resistance would be on both sides, so only one side would get the (Fill in the blank) and this would rile the other side up, causing a rise in incidents, which is what could lead out guys into whatever mess is about to erupt. 

Yes, I have the idea of making new species, but this would be done to supplement already existing species that we like and would want in the story. So, we'd pick the species we like from existing one, divide them into the two groups, then add our own creations to either side as we see fit and hopefully provide a balance of power. Ex. Daywalkers got dragons, and not much can argue with the kinds of dragons I like RPing with :P

As for concealment, I would see base level enchantments that each side can learn at a certain age, but p until that point they have to be hidden away, or the family lives in the countryside until the child can use the concealing magic. But that's only for the non-humanoids. I'd say the human shaped ones can also use the concealing magics, and do, but they also can just put on the right clothes if they are feeling daring. I was also thinking of establishing a few places kinda like the MoM from HP, which would be safe places for the non-humans to just be themselves. Like bars and hidden playgrounds for the kids :P

I always like feedback on the ideas I pose, in case my partner doesn't like them or have a different idea. The only thing I can see about you asking a lot of questions is that I may feel eventually that I'm taking too much control in the set-up, so please be sure to speak up if something I put forward isn't what you want, or if you have ideas of your own. I too like posing questions, and will do so like this:

Are there any particular cliches you'd like to avoid? 

Any particular races you definitely want/don't want?

I remember you saying something about not doing modern. For me, using a semi modern earth setting as a base and building from there, or any base really, is a good way for me to start cobbling together my worlds. So, if not a modern base, what kind of base do you want? The world we come up with may end up completely different from the base, but a baseline helps my thought process. :P My own psychologist(I am a psychologist by the way) tells me that my thought patterns, when left to roam go, "From A to B to. . .L, and Z!" when I'm communicating to people my ideas. We start off talking about carrots and end up with me spouting off something about the Indy 500, which will make perfect sense to me, but no one else will get the connections. . . .And I ramble :P So yes, a building block, can be as simple as one of my favorite ideas "Everything is powered by a very interesting kind of plant that glows" That world looks, so awesome. . . . .