Hmm ... but again and again I'm reminded of the practical issues surrounding such a play-style. 
How would the Inquisition get funding? Presumably through your disillusioned zealots or through conquest.
Who would be your army? Surely large portions of the Inquisition cast (and the forces themselves) would have nothing to do with you if you proved yourself a Tyrannical Despot, which of course could only start happening after the fall of Haven (you'd have to play nice before you were put in charge). Cullen, Mother Giselle, Sera, Verric and perhaps even Dorian and Blackwall would hate your for being a Tyrant. Lelliana and Cassy would hate you for disavowing and attempting destruction of the Chantry. Solas will hate you for playing at god. Josey would hate you for being a warmonger. If the hordes of people leaving around you were too severe and threatened failure Bull would advise the Qunari to deal with things themselves ... so that just leaves you with Cole?
Many of them may even try to remove your from power or assassinate you themselves.
If you still had sufficient forces (that you could not initially fund nor feed before you began your crusade/conquest) how would you then deal with the combined forces of Orlais (admittedly weakened by Civil War) and Fereldon? Would they put aside their present issues to clash against a religious radical and his horde of raving zealot psychotics + mercenaries such as the Freemen of the Dales first? If you were deeply entrenched in your conflict against the Chantry, Orlais and Fereldon (literal war or not) would you still have the resources and man power needed to also deal with Corypheus? That is an absurd amount of fronts you'd be fighting on (literally and figuratively).
It seems like in order facilitate this type of play it would require "Darkspawn Chronicles" levels of retconning to achieve ... 
As I mentioned, a big part of this plan would be to get nobles to follow our new religious sect, which like scientology, requires significant donations of cash or troops or supplies. Tribute to the living embodiment of Andraste's will who will prevent the Breach from swallowing the world.
We could make the ruler of Orlais be our puppet similar to the Briala-Gaspard option currently available. We would have to do a lot more than eavesdrop and find notes lying around, we would have to find some truly damning evidence to keep a powerful Orlesian noble under our thumb. It could even be Florianne to ascend the throne, if we convince her to abandon Cory and follow us, if we clear the path to the throne for her.
For the Grey Wardens, we already have the option to have them remain in Southern Thedas and help fight Corypheus, so rather than leave them as a war table operation, we conscript them as we did the templars/mages.
I think you're seeing this as a complete 180 to what's already in the game. I don't want the Inquisitor to grow a twirling moustache and cackle as s/he beheads all opposition. This is a more subtle difference, where the PC can claim to be seizing power in order to save the world. It's basically why the Exalted Council is formed in Trespasser, anyway; everyone is afraid that the Inquisition has gained too much power and influence and needs to be controlled/disbanded. Just in my scenario, their fears are more justified than they are as the game exists now.
I just don't see the point of this. What would "slandering" the Chantry mean when it's still going to be around afterwards? What's the appeal in playing a ruthless Inquisitor when you lose your powerbase in 2 years?
As others have posted, being able to predict the future is not a reasonable justification for our actions. And who is to say that if the base game was dramatically restructured to allow ruthless options, the Exalted Council might not end differently as well? All of what I'm saying is assuming that the game doesn't remain identical except more renegade responses to the current paragon options.
At the time of the game's release I didn't, but now I just accept that Inquisition is about restoring the status quo in Southern Thdes, learning important Elf and Dwarf lore, and setting up the Qunari-Tevinter War for the next game. That's all that's really relevant.
You're plenty capable of being ruthless in your choices and dialogue already. Given the lack of a real ability to alter the south, I acknowledge the merit in adding more roleplaying options as long as that's clear.
None of the Inquisitor's dialogue struck me as particularly ruthless. Yes, we can conscript templars/mages, banish Wardens, and allow Celene to be killed. But the three dialogue options never had the emotional resonance of being an a*hole. They were all framing the identical response in slightly different ways.