A taintless Grail war is a good idea. As is one that uses something wholly other. After all, one of the ideas I wanted to use involves something only slightly related to the original.
And Grill War isn't that bad an idea. I've actually had a similar thought. Rather, what would happen if some of the Lair found themselves summoned as Heroic Spirits in the 5th War with the standard Masters and Gil running around? Of course, they'd have capabilities stemming from some 2nd Magic spawned madness where alternate versions of themselves have become HS's but still, not so far off.
Imagine Mike being summoned by someone other than Sakura. No. Really. Just think about it.