KOTOR has always been a highly 'mod-friendly' series with a still thriving community despite the lack of any official tools because of its foundation. The Odyssey Engine (used in KOTOR 1 & 2) is simply an updated version of the original Neverwinter Nights Aurora engine, which we all know was build mainly to allow players to create their own mods with its toolset.
Now, no one really knows if the Mass Effect engine is configurable because as far as I know, they havent released the tools to do such mainly because Mass Effect has only appeared on the Xbox 360. So far, most (if not all) the major mods for the KOTOR series has been for the PC versions due to the availability of tools and platforms that are simply lacking in the Xbox versions due to the fact that we users/players don't have access to the Xbox programming code (as far as I know but I'm hardly a programmer so I could be way off).
So a number of factors will determine if the mod community will thrive for a possible KOTOR 3. The number one factor is the graphics engine and easibility of developing for it. Now I know that someone in the community will find a way regardless (if it ends up on PC) but ambitious mods will only come if some tools are developed. And note that having a better graphics engine doesn't mean it will be easier or better to develop for as the lack of fanfare for modders on NWN2 as compared to the original NWN has shown.
"We know the Answers..all the Answers. It is the Questions that we do not know..."