There are plenty of games that keep the same PC throughout and this doesn't seem to affect new players much, it all depends on how this previous PC and an already set world+lore are presented to the new players, and how much the game itself compells them to learn more about it (gods know i lived on the wiki). I'm talking The Witcher here for example, Geralt for three consecutive games, i started with TW3 and had no problems. Same with Dragon Age, my first was Inquisition and i got acquainted with previous characters and a whole lot of lore and history from the first two games just fine. Sure, for older players having to read about dalish elves, templars and circles again might have been a bit repetitive, but all those codices lying around did their part for me and other newcomers to the series. What about being a new player to M3? Shepard has a lot of background to catch up to. New players adapt to characters with a background from previous games just as they adapt to new menues, new worlds, new ui, combat styles. BioWare's dialogue wheel, even.
A default world state was already given for Inquisition, in relation with the Warden, Morrigan, Hawke, Varric.. Remember we have the Keep now, and just as DAI offered a default world state or the option to use a custom one, DA4 will most likely offer the same. With such an option new players could get a default world state and those familiar with DA can load a custom state from the Keep.
Personally i'd prefer the option to play as our (former) Inquisitor or a new protagonist, but i'm well aware that would imply a lot of work and is too ambitious so it's most likely not gonna happen for DA4.
All things considered, i lean more towards seeing and playing as the Inquisitor again, as my canon Inquisitor was a Lavellan who romanced Solas, playing as her on DA4 -granted, if DA4 is about Solas and his plan at all- would make the experience much more interesting and worthwhile for me (us solasmancers do need some closure, y'know).
I'm not against BW pushing the whole Solas situation a bit for later, and have DA4 be about something else, ie if DA4 were to focus on the Qunari vs Tevinter situation, while Solas quietly continues to collect elves in the woods as a background minor detail that then a hypothetical DA5/dlc would deal with, finally closing everything up.It's just an option, but again, not one i see BW going for.
As for the Inquisitor having lost an arm, and how to start a new game with an old PC on noob mode.. Solas only delayed the lethal effect of the mark, he didn't stop it, so as the mark continued to expand and being a threat to the Inquisitor's life, the arm had to be amputated. I do think the amputation was in order to make it clear there would be no more rifts, not to cut the Inquisitor off from future games entirely. I agree with many comments i've seen here and in other threads, prosthetics in a world of magic, runes, and Dagna? Sign me the hell up. Sera's endscreen if anything proves that having lost one arm doesn't necessarily mean the Inquisitor's adventuring is over. Losing a limb doesn't really stop you from living, implying that because now this character is one arm short now they're useless is an insult. A magical prosthetic arm could do wonders, no matter what class the character is, and it'd even add more fun to crafting IMO. How could someone as experienced and skilled as the Inquisitor mysteriously start at lv1? That detail can be easily solved by having them go through new training to adjust to the prosthetic arm, they'd have to learn old skills all over again because now things are different for them. It gets a bit tricky for mages, because knowing spells have little to do with having any number of limbs, but depending on how the prosthetic arm works, what materials it's made of, what sort of runes Dagna can make for it, i'd allow a halfassed explanation of how an artificial limb of such qualities messes the flow of magic from the Fade so a mage Inquisitor has to learn new ways of casting the same old spells.
Either way i'm not holding my hopes up, is just that having my Inquisitor back would really make it for me, and my sad and betrayed Lavellan. If we get a new PC and the Inquisitor is mentioned or shows up for a cameo at some point, i'd really prefer it if it weren't so cheap a treatment like the Warden's on DAI. A wartable mission, a couple of mentions, and not much else? That'd be terrible.