Yes, there has been precedence for a lot of what people are asking for. But as you say, DA and ME are different series with different structures and (more importantly) different expectations from the fans.
Quite honestly, I think the marketing for ME3 as being "the perfect entry point into the series" was such bull, and just a method to increase its consumer market. There is no way that any fan of the series as a whole would say that the last game in a trilogy is a good point to dive in. I am playing TW3 right now (I am NOT comparing the games here) and I can tell that this is the third in a series, as a lot of characters and events from the previous two games are referenced that I know I'm missing. I don't think ME3 would have made much sense to me if I hadn't played the first two games, or at least not sufficiently for my narrative-focused taste in games.
You would think that, right
I mean, there is all that pre-established lore, hours upon hours of character and plot development - honestly, who is going to jump in at Game Three? Still, most of the people I knew in person at that time who also played Mass Effect were more casual gamers than myself and they told me they either only ever played ME3, or, if they did play the other games, became interested enough in the world that they sought out the previous installments only after they played it. Although I have no idea how telling that is because one of those people only ever played MP seeing as, and I quote, "there was too much talking in the SP campaign".
Oh and do not get me started on The Witcher
I am currently in the process of playing TW3 (which will be put on hold starting today until further notice because nnngh Fallout), and not only is there stuff from previous games, there is stuff from the books. I just got to Novigrad, and Geralt made this off-handed remark, that made me do my best impression of a confused dog and go "huh?" It must have taken me a full minute to register that wow, I actually remember that from the books.
I was actually surprised that Cory became the main villain of the game, that was fairly unorthodox for Bioware. When we got spoilers that he would be in DAI, I thought he would be the mini boss of the Warden quest line, not the main game boss. And who knows what plans Bio had before the Exalted March was canceled. I think that put a lot of the plot for the overarching series into disarray. I've posted previously that I wouldn't be surprised if Cory was meant to be a mid-game boss and Solas the end game boss if Exalted March had seen the light of day.
But as I mentioned above, I think from a consumer service perspective, which I am fervently hoping Bioware still has, the general audience of the DA games now come to expect a new PC for each game, as they expected to play Shep for each of the ME trilogy games. I think it's a small but vocal minority who want/expect the Inquisitor back, and fulfilling their wishes risks upsetting the rest of the fans who want a new PC, or new consumers who would better connect with a new PC with no baggage than to the previous game's PC. If the Inquisitor returns as a PC, we will have people shouting "why wasn't my HOF or my Hawke a returning PC?! Why the Inquisitor?!" I don't know that Bioware is willing to risk that backlash, especially when (to me) Trespasser seems to be an easy send off to the Inquisitor.
I understand where people are coming from saying it would be better story-wise to have the Inquisitor keep the antagonism with Solas, but looking from a game development and marketing perspective, I think it would be more harmful than beneficial. And hopefully Bio can still find a plausible method of introducing a new PC with a strong relationship to Solas and as strong a motivation for wanting to stop him as the Inquisitor does.
As regards Cory being a mid-game villain - I think there is a recent interview with David Gaider where he says something to the effect that Inquisition was initially supposed to be about twice as long as the final product was, and that it was split due to production constraints. I have yet to hear the whole thing, because it is very long, but it is my understanding that the portion that never made it into Inquisition is around there somewhere as something of a groundwork for a possible next installment. I have yet to hear the whole thing (the interview is quite long) so I may come back and correct that assumption.
But I see what you are trying to say about consumer perspective. Personally, part of the reason why I am still debating this issue is protagonist whiplash; each new installment brings yet another nobody who works their way up and saves the day, only to bow out just as things are starting to heat up - there is a disconnect there, this jarring jolt, like you are sitting in a car, enjoying the ride, and the driver suddenly jumps on the breaks. And this keeps repeating every hour or so. But I did say "personally" - this is an opinion that is mine and mine alone. But, at the same time, I can see how a new player contemplating the purchase of a DA game might be put off by a returning protagonist.
However, it occurs to me - if we go by the logic that in DA, it is the setting that is the main character, and not the protagonist, would that not be the similar situation as with ME3, where a new player would be jumping into the shoes of a pre-existing main character? In both, there is accumulated lore, you would still need to explain the whole Elven gods thing, why this Solas guy is suddenly important and why his actions are a big deal, and then you have the Chantry and the conflicts therein, the dwarves, lyrium, red lyrium, mages, templars, Tevinter, Qunari - regardless of whether there is a returning protagonist or not, you still need to re-establish the entire setting, the tons of existing lore and previous developments that returning players know already, but the new ones do not. So, looking at it from the consumer perspective, if they are picking up Game Four, they do so at least suspecting that there is going to be some accumulated baggage and that they would have to play catch-up with all the things that have happened anyways.
Not to mention the fact that the most likely main antagonist is going to be a returning character. And, if the Inquisitor is only made interesting because of Trespasser, the same could be said of Solas. Until the endgame of Inquisition, he was just another companion, with the reveal of his identity afterwards elevating him to the status of a "thing that may pop up some time down the line", much like, say, Flemeth. But it was Trespasser where he came out and cemented himself as the focal point of the greater upcoming conflict, and revealed his motivations for bringing this conflict about - the very motivations that make people view him as sympathetic and more than just a run-of-the mill Big Bad. If Solas is seen as an interesting and multi-dimensional antagonist, it is only because of the developments of Trespasser, which makes him no different from the Inquisitor who was made interesting and multi-dimensional on account of the same.
But, as I said, I see and acknowledge your points. In the end, though, it does not matter what they do - somebody will always end up getting upset.