According to monthly manga information magazine "Pafu", Arakawa-san accepted the interview and mentioned like this;
"Shanghai" will start after the end of the movie and whatnot (maybe FMA)
There are plans of "one-shot manga" and "short serialization".
I guess the new series of Sunday means "short serialization", but "Pafu" was sold before the official announcement of Sunday, so there is no detailed explanation about the new series like the title and the length of series.
Sunday doesn't say "short term" in the official announcement (purposely?), so I wonder if readers will accept a situation.
But there was a case the work that was originally planned as the short serialization changed to the long one by a lot of requests from readers, it's "Ushio and Tora". Then there will be also its possibility about Arakawa-san.
The following is the journal on February 27th
Yey! Xing, Homunculus and Ishval!
The announcement of new cast had just finished on twitter of the festival! I read it in real time.
The cast is Miyano-san (Ling), Nakamura-san (Greed), Miyake-san (Scar) and Goto-san (May) so far.
I hope Ling who acts together with Ed and Al will be in the movie at least...!!
I'm sorry, I assumed mistakenly that the cast of festival is the same as the movie because the festival is the one for the movie.
And the festival seems to have the themes, the one in the first day is "Ed's journey", the other one in the second day is "Al's trajectory", and the festival will be the content that cast looks back the whole 64 episodes of Brotherhood with live dubbing and performance from Ed's perspective in the first day and Al's one in the second day.
The following is the journal on February 25th
It found the new serialization of Arakawa-san will start in "Weekly Shonen Sunday 19th issue" that will be released on April 6th.
The information is from the advertisement of Sunday. [link]
The title and detail were unknown so far.
She was a reader of Shonen Sunday in her childhood, then having the serialization in Sunday might have been her dream, but I was so surprised because I have believed her new work will be in Gangan. What is going on with "Shanghai Yoma Kikai"...?? Shanghai is the work of Ganagn, and it's true she went to Taiwan for the research of Shanghai with Gangan related people. Then, I wonder if she'll draw it in Gangan, the another new work in Sunday. Or she'll draw RAIDEN-18 in weekly Sunday?
I seem that it might be difficult to realize she draw two works at the same time because Sunday is weekly magazine, but I feel she can do it with her toughness.
Anyway we must wait for more information.
Addition: I found the article about the new series at the website called "Mantan-web".
According to it, the editor of Sunday approached her and the concept emerged, then the content fits teenager-oriented and weekly serialization, and it was decided.
The managing editor seems to say "The content just fits Sunday. I'd like Arakawa-san to draw her own unique theme and characters".
By the way, I tried walking around here and there, then I reminded that Shogakkan that is the publisher of Sunday has the problem of editor's quality...there was the trouble between the publisher and Raiku-san who is the author of "Zatch Bell!", and I read about the bad habit in the whole publisher.
I do hope the constitution of company will be improved, and the editor never squashes Arakawa-san.
The following is the journal on February 22nd
It found the new cast of the festival will be announced at CM of festival this weekend!!!
Maybe the commercial will be aired within the airtime of animation program that Brotherhood was ever aired.
I'll report in this journal as soon as I know!