Mighty Avengers #21

This story will last 3 issues and the covers are linked to give 1 large image of the whole team.

#23 will reveal that this isn't really Scarlet Witch. Her real return will be in Avengers: Children's Crusade where the possibility will be raised that the version Beast and Hawkeye met earlier (Endangered Species backups in X-titles and New Avengers #26) was a robot created by Dr Doom.

Mount Wundagore was the site of the High Evolutionary's castle where he uplifted (to use David Brin's term) animals into his New Men or Knights of Wundagore. Bova was an uplifted cow who acted as nursemaid to baby Pietro and Wanda who in the origin accepted at this point were the children of Magda, estranged wife of Magneto.
But Wundagore is also where evil Elder God Chthon was entombed. He wrote Marvel's premier magic book the Darkhold. Scarlet Witch's magical abilities were attributed at this time to her having been born on Wundagore
The twins learned this version of their origin in Av#185-187 when Chthon got Modred the Mystic to provide him with Scarlet Witch as a host. It seems they're trying the same trick with brother Quicksilver now.
It was after this that Bova nursed Modred back to health. He's had lots more adventures since then both as hero and villain, fighting the influence of Chthon.
Bova was last seen in the 1st Quicksilver series where she was a recurring character through to the crossover ending with Heroes for Hire. Pietro worked with the High Evolutionary and his New Men then in Wundagore.
Chthon most recently tried to return to Earth again in Mystic Arcana: Sister Grimm.

The Young Avengers are fresh from their Dark Reign series where they faced some less-than-heroic contenders for the group title, and Norman Osborn and some of his Dark Avengers.
Since then NO and the whole of DAv have been in All-New Savage She-Hulk #1-4 where the title character Lyra came from a future ruined by Osborn.
Before both of these the DAv were in Free Comic Book Day 2009: Avengers helping Thor against Ymir and the Twilight Sword. And that's where the New Av were last too.

Hercules took over the Incredible Hulk comic after World War Hulk, with Amadeus Cho in tow, and renamed it Incredible Hercules. After SI they were involved in a war between Atlantis and Amazons in #121-125, which resulted in 1 of those temporary rewrites of reality that Marvel is so fond of. During the SI issues we discovered that Cho's coyote pup Kirby had been replaced by a Skrull. While Herc was having a flashback to his early youth in #126 Cho had his own tale in which he located the real Kirby. But he also met up with his old pal Hulk.
Bruce Banner was voluntarily imprisoned by SHIELD after WWH. The next Hulk series starred the new Red Hulk. But soon Banner was freed to become green Hulk and help fight Rulk. They started to share the book as Greenie hunted Reddie, and before meeting Cho Hulk was temporarily turned into a Wendigo in Hulk #8-9 (don't ask). Hulk and Cho parted, but have immediately been brought back together again in this issue.

Since his fall from grace in SI Iron Man has only appeared (as Tony Stark) last issue at Janet Van Dyne's funeral.
That was also the 1st issue featuring Henry Pym since his return at the end of SI. There he learned what happened to his ex-wife Jan, and also everything that occurred while he was 'away'. Then in Av: Initiative #20 Jocasta helped him come to terms with the loss of the woman on whose brain patterns she was based. The pair were then in SI: Requiem where Pym decided to honour Jan's death by taking over her super-id Wasp.
Edwin Jarvis was a prisoner of the Skrulls alongside Hank Pym. Since their release he's only appeared in Av:I#20 as 1 of a group being given counselling.

Mr Fantastic and Thing are here after the end of the Doom's Master story arc ending partway through Fantastic Four #569 (not including Thing's wedding).

Canada's Omega Flight including US Agent weren't seen in SI. Since Civil War they've had their own mini-series and a strip spanning the whole run of the 2nd Marvel Comics Presents anthology book. But of course US Agent is John Walker who used to be the replacement Captain America back in the late 80's. Sasquatch is the only survivor of the old Alpha Flight who perished fighting the Collective in NAv#16. The current Guardian *was* the Collective. And Arachne is Julia Carpenter who was the 2nd Spider-Woman.

Quicksilver also wasn't involved in SI. He's been doing his own (usually dubious) thing since the end of House of M stripped him of his mutant speed power. He underwent the Inhuman's Terrigenesis without permission and gained the power to time-travel. He began treating other ex-mutants with Terrigenesis to get their powers back, with not very good results. But in X-Factor: The Quick And The Dead his Terrigenesis ran out so he lost his time-power but he regained his speed. And he decided to be a hero again. (Marvel's other habit - restoring the status quo.) Next issue's flashback will show he joined the Knights of Wundagore to help them against Chthon.

Before looking forwards I'll just mention that the backup tale in Av Classic #3 has a very different version of how Pym found out Iron Man is Tony Stark, but set at about the same time as this issue's flashback.

Most of this issue's characters will appear in #22 and/or #23 but some will leave here.
Captain America and Wolverine will be back for #23 before joining the other NAv in the early issues of the 2nd Agents of ATLAS series.
Norman Osborn will also be on hand for #23 but his DAv members will split up. Sentry will also be in Agents of ATLAS (with NO), Moonstone goes to Ms Marvel #37 (again with NO) where Carol Danvers appears to die (and the Moonstone MsM will take over the comic for a few issues). Venom won't reappear until CA#48 where he cameos alongside the rest of the team (including NO).
It's #23 also for Guardian and Sasquatch but Arachne won't be with them.The team will split up after this and she'll appear next in Amazing SM #634 where Kraven's family are coming after Spider-people.
Mr Fantastic will cameo in Guardians Of The Galaxy #10 where he is told that Blastaar has taken over the supervillain prison in the Negative Zone. (We saw the beginning of the assault in Av:I#24.) And then he joins Thing and the rest of the team for Nova #21 where Richard Rider is thrown out of the Nova Corps.