can you add support for loading .obj or .3ds 3D models with texture as they are the most common type of model found on the web . (there is not much gltf and the dae to gltf online converter dont work much , i never succeed to make it work with the obj to dae online converters you can find on the web , i wonder if any test was made about it ...) ?
You might be able to use the obj to glTF converter Sean wrote: https://github.com/AnalyticalGraphicsInc/OBJ2GLTF
there => http://chungswebsite.blogspot.fr/search/label/myCesiumflight
or there => https://github.com/chungkn1400/myCesiumflight
if you are interested .
The online collada to gltf converter seems to work pretty well for most people. Do you have an example of a model that is failing to convert?
Maybe useful for others to the list, seeking models to add in their Cesium app.
About 3D models and their availability on the web, I found very useful the function in google sketch
where you can download models from their 3d-warehouse 
then save as collada.dae and convert them with the collide to glTF web app.
The models I used are pretty simple, static, without animation but they works fine for my use case.
My Thanks to the developers
heres a file i converted from obj to fbx then to dae with the fbx models converter i downloaded yesterday
i get “a unknown error has occured” with the cesium online converter (but i succeed with another file c150 cessna)
f14_2.obj.dae (730 KB)