Although he has announced his retirement, there is no rule preventing Spezza from playing if he were to sign a contract at some point during the season. I can see a scenario where Spezza could sign a deal at or near the trade deadline, much like Tuukka Rask did in Boston this season. It all depends on what happens with the group of Marlies vying for NHL jobs next season. As you'll recall, Justin Williams made a similar comeback for the Carolina Hurricanes before the playoffs a few years back.
Kyle Dubas issued a challenge to the young players in the Maple Leafs farm system after the season ended. According to Dubas, its time for them to transition from prospects into every day NHL players. As part of this, Dubas likely won't be filling out the roster with cheap UFAs this season. There is risk to this, if those young players don't pan out, the Leafs will have missed their chance to sign players for their bottom six. Enter Jason Spezza. A mid season signing would make sense if some of the younger group can't hack it. Assuming he keeps in excellent shape, Spezza can still be an effective 4th line winger and he can still be a shooting threat on the second PP unit. If the Leafs need a boost mid season or leading up to the trade deadline, Spezza could be the best option available.
As a matter of fact, it may have been the Leafs' plan from the start. It gives them a little bit of extra cap space to bring up more Marlies players and accrue more cap space throughout the year. The first half of the season would be a tryout. Players like Robertson, Holmberg and Steeves get a chance to sink or swim at the NHL level, if they "swim" they stay in the lineup. If they "sink," Jason Spezza signs a deal and hops back over the boards.
How great would it be to see Jason Spezza suit up for the Leafs one last time? It may be wishful thinking, I may be in denial over his retirement, but I do honestly think it's within the realm of possibility.