While using the preview tool myself for my next project I discovered a helpful feature and just had to implement it immediately.

If you press the left or right key you display the previous or next 3D model in the same directory as the current loaded model. (the graphic above is only a helpful illustration)

I'll for now work with this fast implementation. But I've two follow-up features in mind:

1) Pre-loading some models in the background to enable faster browsing through the directory for big models
2) Some nice sliding animations from one model to the other.