I'll make the lofty prediction that we may see David Miller take a shot at the job within the next year following a grassroots leadership challenge rooted within the rank-and-file of the party. Given the failing fortunes of Canada's social democratic party, we may not see many other "top-tier" candidates go for the job. We may even see Layton run in the same leadership race in such a scenario.
While I think it's in the realm of possibility that David Miller would take a shot at the job, I don't think the federal NDP will grow with another extroverted municipal politician from the city of Toronto. My guess would be it would be better to have a rural, Western politician take a stab at the top job - say, from Saskatchewan or Manitoba. Exactly who that is, I don't know.
Now, I'd like to contrast this with my earlier prediction with regards to the coming Ontario provincial election:
I predict that if Howard Hampton resigns as leader of the Ontario NDP, Olivia Chow will go after it and she'll win. She's got face and name recognition. It would mean for the first time spouses would lead the NDP at the same time (at different governing levels, of course). She would likely gain the support of Toronto mayor David Miller and all those who supported Jack Layton in his bid for the federal NDP.
What I think the Ontario NDP needs is, funnily enough, an extroverted municipal politician from the city of Toronto. If that were the case, with the exception of the bluer neighbourhoods going to the Progressive Conservatives, Toronto would be painted orange.