I would like to see such a lens. The 55-250 is a rather cheap Option, but still quite nice. The 100-300 should sit above it, with some extra Features (USM). The price should be between 1/2 to 2/3 of the 70-300L, with IQ close to it, and a lower weight. I think weight and size will play a role in such a decicion. The Sony Alphas and Fuji all have quality zoom lenses in this area, so Canon actually needs to counter, if they like to sell APS-C DSLRs.
A 100-300 would be a nice option to test the market, for such lenses. But I think such a lens would suit very well on 7D II, and many photographers would welcome such a lens in their kit. A little lower IQ (especially at the Corners), but less expensive and lighter, sounds fair.
I have the 70-200 f4 IS, which is a great lens. I actually like to switch to the 70-300L. The only thing that makes me doubt such a switch is, that in the lens comparison-tool over at The-Digital-Picture, the 70-300 is notably weaker than the 70-200. I know these are synthetic Studio shots of test Charts, and most other test say the 70-300 is very good. I'm just not sure because T-D-P is the only place where I can compare shots of lenses, and not only read test Charts, and These shots tell me, IQ wise, it will be a downgrade. Has anybody expirience with those two lenses?