I think Leliana is great depending on what I think I could do with her. Force change, give people a kick in the balls, propel the world forward quickly. Shake the people's ideas of the old status quo. The world is changing and people are changing, and the Chantry is holding too many back. In other words, I would use Leliana to shake the tree to get more people thinking, a contrasting old vs new way of thinking. But I would hardly consider her to be the one to actually make those long term changes. That would be up the Inquisition and the new Divine who will replace her after she gets murder knifed for the cause.
Viv is awesome for moderate change, which is odd cause she's a mage. But I see her bringing about lots of changes (for mages mostly) but more of the "behind closed doors" variety. She's enough to make people feel uncomfortable and question their views on tradition but comfortable in the fact that she is keeping the mages at bay and the Templars strong. I also think she is the best bet for overall Mage freedom because without power in Thedas, you're not truly free. With Viv, Mages will most likely end up with political power and once the rebellions are squashed then people will most likely settle down while the Mages obtain power in the shadows.
Cass would be my last pick as Divine. I don't even know what her reforms are. It just seems like the same old same old with bits and pieces of change here and there. I can sort of see everything just going back to the way it was. which isn't a bad thing for players who want that.