Plenty of speculation so far about potential successors to Jack Layton as federal NDP leader, but I haven't seen much about potential NDP candidates to follow Jack in Toronto-Danforth, so I'll take a look at some potential NDP candidates for a by-election that will be coming in the next 180 days.
Jack's son, following in his father's footsteps by sitting on city council. Would he follow Jack's steps further and make the jump to MP? It would be interesting, but I doubt it. Layton has been in office for under a year, and has built his powerbase up in Oliva Chow's Trinity-Spadina riding, not the East York area Toronto-Danforth.
City council member for the southern half of Toronto-Danforth and formerly a public school board trustee, Fletcher is solidly a member of the left wing of council, although she barely snuck into re-election in Ward 30 in 2010 over Liz West. Fletcher has name recognition (although not always positive) and plenty of government experience, but the other "formerly" in her resume might be problematic - Fletcher was the leader of the Communist Party of Manitoba back in the day. Considering the communications and image issues the party is dealing with with Nycole Turmel's associations with Quebec sovereignty, the optics of an ex-Communist campaigning to be an MP for an ex-soverigntist wouldn't be the greatest for the party (assuming the by-election is held during Turmel's interm leadership, as the NDP fed council has yet to hammer out the exact time table.)
The NDP alligned councilor for the north end of Toronto-Danforth, only recently elected in 2010. I think she's pretty unlikely to run, but I'll include her on this list just for the sake of it.
Former city councilor & executive director of Greenpeace, Tabuns is currently the MPP for Toronto-Danforth and was a close friend of Jack Layton. Tabuns was the early frontrunner for the ONDP leadership when Howard Hampton stepped down, losing to Andrea Horwath, who has since taken the party significantly away from environmental issues, leaving the former executive director of Greenpeace in a somewhat awkward spot, being forced to advocate for Horwath policies like subsidizing gas guzzling SUV's and voting against the Clean Water Act. (see http://www.blogger.com/www.dirtyNDP.ca for more details) Tabuns running would probably depend on the NDP results in the provincial election, but with a mix of civic experience and environmental background, Tabuns would be a good get for the NDP if they could convince him to run federally.
Former MPP for Toronto-Danforth and a twice failed federal NDP candidate in Beaches-Easy York, Churley would bring experience serving in a provincial NDP government, as well as the attractiveness of recruiting a high-profile female candidate to fill Layton seat (Layton was good at promoting women in politics, something I will always give him a lot of credit and respect for). Churley was also a notable supporter of Layton's NDP leadership bid, which might give her some added credibility. She's currently serving as a justice of the peace, so she would need to give up that respected position for a shot at running.
I'm not the first person to point out that Topp would need a riding if he wanted to be NDP leader. Topp was born and raised in Quebec and has worked in Saskatchewan, but has been based out of Toronto for a few years, so he could use that to fight off allegations of being a parachute candidate. With the timeline for both the NDP leadership and the by-election still up in the air, one interesting possibility would be Topp both running for leader and running in the by-election to have a seat. I remember back during the 2006 Liberal leadership contest a brief flurry of speculation came up about either Gerard Kennedy, Bob Rae, or Martha Hall Findlay contesting the London North Centre by-election, as they were the Ontario based candidates without a seat at the time. Nothing ended up coming from that, and aside from that I'm not sure if a seat less candidate for major party leadership has ever run in a by-election. Be an interesting test of party unity if he did decide to run for both at the same time.
If anyone else has any theories about potential candidates, feel free to leave a suggestion in the comments.