Excellent topic!
I believe there are a few areas that Subaru can benefit, one is the small city-car class, which includes the Yaris, Fit, and similar models. It would have to be hip, maneuverable, and similar in proportion to these, while well contented as these others are. This class of smaller efficient cars may take off, a vehicle would need to be in this class that is quickly growing in popularity, especially for the younger age group including college kids. Perhaps the rumored joint venture with Toyota on a small model is driving at this market.
In the performance market, I believe there is a position awaiting to be filled. As you can see below, strictly within the Subaru market, you can have performance or features and quality, but not truly the best of both.

I believe the Spec B falls short of the pinnacle machine. It lacks the ultimate power delivery, braking capability, and the pull around corners that the Impreza STI has. It does however offer superior levels of refinement in materials, NVH, and suspension dynamics as well as features. A Legacy STI resides in a little-touched class, one that offers the performance and near quality levels of BMW M and Audi S/RS models, but at about 2/3 the price, while featuring a more reliable, efficient, and lighter package; it is something the average person may actually be able to attain in the $35-37k ballpark. I believe that although Mazda has been present in this market and Acura is apparently entering this market with their next TSX model due for 2008 with a turbo 4-cylinder, Subaru could easily complete execution and blow the doors off everything in this market and wake up a lot of much higher esteemed cars. Much of the engineering work already appears to be completed with the large parts bin available. Take advantage of the SI-drive and it's multi-personality capabilities to be able to incur better safety margins and economy for Intelligent mode, while able to shift greater than 50% of power to the aft in favor of a more nimble vehicle. Much capabilities lie withing the system.
Technology and efficiency are also becoming much more hotbed topics and especially with AWD as a standard feature on all models,efficiency is a hot topic. I believe it may be crucial to Subaru's success to adopt a more fuel-efficient strategy including but not limited to:
-Diesel technology
-Direct injection and the capability to run both premium and E85 on higher compression (dynamic and static) engines for greater efficiency, E85 is less efficient on lower compression motors thus I suggest it to be used as dual-fuels for premium-based engines where less efficiency losses are likely to be seen
-Utilizing hybrid technologies much based on capacitor systems to aid in acceleration and city driving where the greatest amount of energy is used, while also being safer for safety and emergency personnel as it is able to quickly discharge perhaps in the case of an airbag deployment or detected crash.
These are just a few of the areas I believe Subaru could benefit. Aiding to efficiency increases, I would be very interested in seeing new technologies and materials that aid in reducing vehicle weight, cost, and complexity while retaining the safety margins.
I believe the Legacy could gain up to 2" in overall length, though it would be nice to see the axles moved closer to the ends, as this does increase departure and approach angles, though the slope angles in the center will decrease. Finding ways to better utilize internal space is important as there appears to be quite a few complaints in the rear legroom area.
Thank you Subaru for allowing us to voice our input. I understand we won't be able to succeed and get special and more performance models until you are able to achieve your goals.