Remember Omen, the demon champion that they cancelled just because they wanted to put in Riven sooner? Well I think that was a poor excuse for cancelling that champion.
A complete misconception, actually.
Take a look at the presentation Designers are Human Too - Causes of Poor Design Decisions by Tom Cadwell, Riot's Design Director. He explains that Omen suffered from a number of major design issues, and "we want to get Riven done sooner" wasn't one of them. The fact that Riven was close enough to done to fill in the gap was more convenient than planned: Omen was already 50% done, and had a lot of work put into him.
There were a number of reasons he was cut. Firstly, not enough of the team was really excited about the concept -- it started falling flat soon enough that it was a warning sign. The character design itself didn't read well -- it was hard to tell if it was supposed to be ranged or melee. Perhaps most importantly, they put a lot of effort into Omen, and they didn't see the sort of quality improvement they wanted: that's an enormous red card in design.
In general, it appears that Omen was, largely, a conceptual failure. Tom basically calls it a character that someone should have said "no" to the presented concept of far earlier.
Did it maybe have some potential in a different form? Maybe. But the effort needed to take it there wasn't worth the time, energy, or expense given the development path Omen seems to have followed.