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It will be interesting to see. Leica discontinued the 35-70 2.8R due to difficult, expensive, and slow production capability which (probably) did not make it profitable. A 28-90 2.8R would likely be more expensive! I agree with Dirk that likely a 28-90 in the 3.5-4.5 aperture range is more likely.
By the way, the 10/03 Popular Photography has tested the 21-35 R zoom, and said it is likely the best lens in this category they've tested.