While this is all conjecture, Stamkos definitely has gas left in the tank, putting up 84 points last season and over 100 two seasons ago. He still has his goal-scoring touch too, with over 30 goals last season.
Stamkos has won 2 Stanley Cups with Tampa and likely has some strong personal connections with the City as well after playing his entire career there. However, if he does decide to see what his worth is on the free market, Boston could be a great fit, especially after losing big-time goal scorers in Bergeron and Krejci. The only determining factor will be whether or not Stamkos wants to pack up his life and move this late in his career.