SureAI

With Forgotten Stories coming soon, I am wondering about what had already been announced in regards to replaying with FS (i.e., starting a new game).

You mentioned a secret ending. Is this similar to the late-game branch that exists in the base version?

You also mentioned new quests for the Veiled Woman as well as the Undercity. Those will be Side Quests?

Will playing the Main Quests be similar to the way it is played now (apart from the secret ending) or will you have added/changed some of those quests? In other words, could we expect playing the Main Quests in FS be a new experience or not?

I believe it's recommended to play a new game with this. I know that I'm starting from scratch just so I don't miss anything

They have been trying not to reveal too much, but I am pretty sure based on Dev posts seen so far that the "secret" ending will not be as simple to find as getting to a branching point and then making the proper choice, like you can do now.

They say there will be hints and clues to help us find the path to the new ending, and there is no way to tell how early in the game those hints will start to be offered, so it is definitely advisable to start a new game. It has also been suggested that many players might not pick up on the clues and miss this ending. They are not calling it "secret" for no reason. It kind of adds a new puzzle element to the game that was not there before, but those who have already finished it once should have an advantage in catching the different dialog.

I've been translating the German forum since this one is too quiet and found this:
I guess that confirms part of my questions.

Yeah, not only the secret ending gets added, but sidequests that interfere in some ways with the main quest line. Please don't quote or paraphrase the following sentences, more experienced Skyrim mod authors and actual programmers (think of people like Arthmoor, DavidJCobb, Mator etc.) can probably put it in better words.
The creation engine is known for putting a lot of stuff in your save games e.g. script properties and such. It's pretty risky to change these values around during a playthrough.
Let me give a simple example of what I mean: We changed these values here and there in a couple of our patches with the sidenote: "This only works for new saves, where feature xyz is not enabled yet.", e.g. think of the patch where we moved the fast travel marker for the South Quarters closer to the myrad tower to shorten the overall travel time etc. - this worked for new save games, in which the player neither entered the South Quarters nor enabled the fast traveling system in Ark while entering through the front gate with Jespar. But old save games saved the old position of the fast travel marker in the South Quarter, since the script and the reference of the marker were already initialized, hence the player still got teleported to the old marker instead to the new position / the player wasn't teleported at all.

And this happens in a much larger aspect when FS is released. So stopping a playthrough, to wait for FS is not really worth it - even if you only have little time to play games - it's simply not worth the risk to run into "newly added bugs" because the engine can't handle certain changes that well. We put a lot of thought into having so called Failsafes in case people install FS mid playthrough, but some stuff just can't be thought of or is way to complex e.g. the new class "The Phasmalist" which is probably more complex than the other skill trees combined. We have a couple of testers right now, but they all test with new save games (= started completely new playthroughs).

tl;dr
Starting a new playthrough is heavily recommended. We don't have the time and man power to make sure that Forgotten Stories works with all its features when it is installed mid playthrough. Though, we try our very best.

Edit: Wording