Assimp use its own structure to store the datas (textures, vertices, indices, uv, etc...), but most of the application have their own scene graph or structure !
It will be fine that assimp use the 'Proxy' pattern, this way :
int GetIndice(int index);
vector3 GetVertice_P(int index);
uv GetUV(int index);
This way it will be possible to create our own proxy and Assimp will play directly with our structure/objects/graph !
It is usefull for hughe scene because with the current version we have 2x the data in memory, the Assimp one and our own structure that we copy.
Also, once Assimp has imported we have to convert it !
Of course, Assimp can provide its own default implementation of the objects (like today) based on the proxies.
BTW, I just describe the basic principle but it has to be well done to support all kind of scene graph etc...