It's nearly impossible for me to properly tell you what you want, without giving specific mission names. In fact, it seems Sanunes just did ^
Anyway, I'll try. But some mission names have to happen to properly explain.
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After main campaign:
-No, there is no true 'Continue+' like ME2 had.
-Instead, you are brought back a couple missions to pre-Cronos Station.
-From that point, you can do any of the DLCs in case you haven't already, and any outstanding missions or side-missions too.
-Or you can opt for a NewGame+ which hands more weapon and mod levels at you, but also unlocks the VI-X ranks for weapons (can be helpful for Insanity achievement).
From Ashes:
-Mission, full squadmate (a kind of offensive adept instead of crowd control Liara or defensive Kaidan), unique cutscenes and Normandy dialogue throughout game.
-I do it as soon as possible (once I get ship freedom). It can happen at any point, but the later you do it (despite the slight plus of a bit of unique dialogue lines, aka Liara in ME1), the more pointless it is.
-If doing a 'main playthrough', do it ASAP, but if RPing a specific type of Shep, it's up to you, and even up to you to even go for it.
-If opting for 'full playthrough', it may be good to bring the squadmate to Thessia mission, as there is more unique and fitting events there that can involve him.
Extended Cut:
-Changes parts of final mission, adds/modifies things based on EMS, alters the presentation of the endings and gives extra content to them.
-Changes key dialogue, new ending added.
-*Personally* would suggest, if you have the time for it, to do a first NewGame without Extended Cut installed at all yet. And then a NewGame+ (or at least Continue+) with the EC installed, just so you can see the differences like everyone else did and raged at.
Omega:
-Forced 4-mission line, temporary semi-hub, temporary squadmates (adept and sentinel).
-Best done at middle of game, right after 'Coup' (you'll know it when it happens) imo
-But also find it to be okay at many parts of the plot, as long as there's a lull in things; particularly the 2nd half of the game though.
Leviathan:
-Free 3-mission line, temporary semi-hub, slight involvement of all squadmates.
-Best done in one of two ways...
a)Spread out through second half of game; the first one shortly after Monastery, the second one before Thessia, the third one before Cronos but after Thessia.
b)Done all at once before the ending stretch, so either before Thessia or Cronos.
Citadel:
-THIS ONE IS COMPLICATED.
-Forced 4-mission line, more true hub, possible temporary squadmate (vanguard), casino gambling, arcade games, arena (many temporary squadmates), more social events, social party; most of these things indicated in the trailer itself anyway.
-First off, like Extended Cut, I'd actually suggest NOT playing Citadel the first time around, then either for a NG+ or Cont+ see it again. For a Cont+, you won't be able to spread the content around though.
-Mission line could be done whenever there's a break in the plot (like Omega and Leviathan), but in this case I'd particularly say to wait until you have all the party members you're going to get (max 6-7, min 3), as they all get involved.
-Fill out the time between the mission line and the party with visits and main game/other DLC missions, as each count as a break that you can get notifications to visit characters. Alternatively, Arena matches (with their own missions) count as a story mission, so you can go back and forth between Arena and visits until you get all of both done.
-Once fully done all the side stuff, and right before Cronos, start the Party. You can do the Party well before Cronos, but doing it right before Cronos does ensure that everyone who could attend the party, *is* at the party. Once you understand how the plot and party works for future playthroughs, you can play around with the story configuration more.
-Like I said with Continue+, this stuff could technically all be done at once before Cronos. It's especially jarring to the main plot here, but fine.