That's not an accurate description of either game. The HOF Blackwall'd the entire way through DA:O, which is to say just had people assume you were a GW (which was only technically true in the sense that you survived past the Joining). The treaties came up exactly once (Orzammar). Otherwise, the GW achieved everything by (1) having people take you at your word about the Blight for (unexplained reasons) and (2) being so awesome at murder that you could murder your way through every political instability problem facing your supposed allies to horse-trade for an army.
The HOF is a person who (1) was awesome at killing and (2) was lucky enough to deal with problems that otherwise resolved themselves after every obstacle was thoroughly killed. Hawke was the reverse, in the sense that (1) Hawke was awesome at killing but (2) every problem exploded horridly after all the obstacles were thoroughly killed (because generally Hawke always arrived too late rather than on-time like the HOF). Exact same set-up, only Hawke faces a plot of less cataclystic significance and Hawke fails through power of plot instead of succeeds.
The Inquisitor is also a person who resolves every problem through thorough killing, only you're right that this time having the Anchor actually made the plot possible (because it was a condition precedent to closing the breach/rifts and made the Inquisition possible). However, the Inquisition only gets formed because (1) you're kind of unkillable (e.g. mountain faills on you, don't die) and (2) you're super awesome at killing.
Almost anyone can be in the same shoes as the HOF. I mean, Hawke is indistinguishable from the HOF other than Hawke failed because of factors outside of Hawke's control whereas the HOF succeeded because of factor's outside of the HOF's control.
Sorry to just jump into your conversation, but I really do find it interesting
! I agree with your conclusion if you rephrase the initial statement. Any PC can be in the same situation as the HOF and succeed as long as they are an exceptional combatant ( Hawke, The Inquisitor ). It's made perfectly clear in each origin story that i've played ( Human Noble, Circle Mage, Dalish Elf ) and each game that the PC's martial prowess is in a league of his/her own.
For example: In the human noble origin Dairren comments on how the PC rekt him in the last tournament handily. Fergus mentions that if he was a Warden he'd be looking at the PC to be the potential recruit. Howe comments on you being trained as a warrior, and then just before the escape Duncan comments on the PC fighting their way through all of Howe's men in the castle to get to him. As a circle mage Irving mentions that the PC's harrowing was the fastest he'd ever seen. The Dalish and Duncan mention how it's astonishing that the PC survived the taint and is still able to fight for as long as they have ( all the way through Ostagar to the joining ) when so many others succumb almost immediately. It is heavily implied that the PC is already a badass, not just some average guy/gal.
To address #1. People of importance knew the HOF was a Warden due to their arrival at Ostagar with Warden Commander Duncan. This included Loghain and King Cailan as well as their guards, soldiers, and anyone else that survived the battle. Not to mention immediately after the battle Loghain spreads pictures of the HOF in order to have you hunted down. Even in Lothering while recruiting Leliana the PC is already being hunted. Hell a lot of people in Ferelden already know the PC is a Warden. So, I disagree on this point, the HOF wasn't "blackwalling" at all.
As far as people taking you at your word that the blight is coming...the game explains that along the way. The humans know the blight is coming because their king and part of the army got owned by the darkspawn at Ostagar, then Lothering gets rekt and as you progress through the game the blight spreads. It's mentioned throughout the game ( especially at the landsmeet ). The dalish ( Zathrian ) can sense the blight, and the dwarves mention something about the darkspawn behaving differently.
Anyway yea, the game itself goes out of it's way to make it known that not just anyone could have done what the HOF did. Same with Hawke in DA2, and the Inquisitor in DA:I. I agree with you that the HOF succeeded in part because of factors that were outside of their control, but their own ability, diplomatic skills, and luck played a huge part in their success as well. Omitting this in an argument kinda makes it seem like you're begging the question. Just sayin.
With all of the above crap being said, Dai makes a very good point that the devs are going to have to get creative if they want to sideline all of the extremely powerful people in Thedas in order to make a believable scenario involving a new PC. Conversely they could make you start as someone in a position of power who has some form of credibility, so that the powerful factions of Thedas would be more inclined to support the new PC. I'm sure the writers can come up with something.
As AlanC9 already stated, we all know the drill.
HOF will conveniently be sidelined with a codex entry saying that he finally pierced the heavens through sheer force of will and one shotted the maker. Now he's kickin it as the ruler of the Golden City and worshipped as Tiber Septim II. Hawke will be off nailing his/her LI, and the Inquisitor will be off inquiring about things. The moons will align and the next PC will be just as badass as the other three and get the job done xD.
Knowing this i'd rather have the option to just play one of the current protagonists in the next game, preferably The Warden. I'd also like the devs to avoid sidelining people who should be present in whatever story they come up with. I'll keep dreaming lol.