If you only need a rear seat as roomy as that in your current car, few four-door cars are ruled out. I used to have two boosters plus a car seat in a Mazda Protege5.
Since you like to hold onto a car forever, Toyota and Honda tend to be the best candidates. Unfortunately, neither has offered a wagon recently (unless you count the Acura TSX, now discontinued).
A Toyota Prius v might be a good fit. Very good reliability and fuel economy, especially if driven to maximize the latter. Likely inexpensive to insure.
The Honda Fit hatchback might have enough cargo area for you. Worth checking out.
Beyond that, you're looking at a crossover. The new HR-V is essentially a Fit with a higher ride height and optional AWD--no extra space inside. Next size up isthe CR-V and the Toyota RAV4. For 2016 the latter will be offered as a hybrid.
The Mazda CX-5 might also be a good bet. But as with the latest Subarus it's too soon to say how it'll hold up after 10-12 years.