@psandak: Interesting--I'm not familiar with WoW, so I don't have its systems for comparison, and I'm not sure which other features it may have that might make this more challenging. I'm basing my assumption of the "relative" ease of implementing on the existing ability to add extra slots and tabs to inventories and cargoes, and the fairly quick, painless, and bug-free implementation of the guild banks. As a programmer, I know that it's always easier to add features that use mostly existing code than to add new features that require a functionality not previously considered, even if that feature change looks small. Which doesn't mean adding Outfits is easy, certainly! Just...more bang for the buck. I'm glad you find some merit in it!
I can tell you right that ALL the inventory and cargo space that becomes available when you pay credits for it are already there they are just hidden.
I guess it really all depends on where this (effectively) new inventory space would go in the interface and by association the database. However, even if it is simply added on to say the character inventory interface window, it still may be difficult to add.
With regard to WoW - first of all inventory space in that game is based on "backpacks". Each character starts out with a single 16-slot pack and can add up to five more backpacks. the character bank also has a fixed number of slots and the ability to purchase additional slots for backpacks. now here is where it gets a little confusing. generic backpacks had an original maximum of 14 slots (I'm talking about when WoW launched now current). However, there are also item type specific backpacks. Loosely translating to SWTOR parlance - they had a backpack for Bioanalysis materials, and a pack for scavenging materials, and a pack for archeology, and for UT, etc. These packs had a larger maximum capacity but they could only hold a specific set of items - if you tried to put a piece of gear into one of these the system would tell you that it won't fit. I believe the maximum for these bags was 20.
As to its "outfitter," the add-on (created by players), allows you to save the outfit you are wearing. You can save multiple outfits and when you select one in the add-on's interface it swaps what you are wearing for what the outfit has saved. Additionally an "outfit" does not HAVE to be a full set of gear and you can have multiple sets of gear on at once so long as none conflict; if there is a conflict the last outfit chosen takes precedence.
After many years of having this add-on and a LOT of players using it, Blizzard developed their own "equipment manager". It had some problems - 1) an outfit was a full set of gear and you could only have one "outfit" on at a time 2) in order to load an outfit you had to drag an outfit icon to a hotbar slot. Even in WoW WITH a macro system to effectively stack abilities into one slot, hot bar slots are at a premium. Eventually they added a button to the character sheet that opened a menu for outfits.
However, the outfit system did NOT include extra inventory space, which is why there are a lot of threads on the WoW forums consistently asking for more capacity.