What video formats/codecs should a video editing software aimed at beginners/semi-pros definitely support
To be frank, beginners and semi-pros will be capturing their videos on smartphones or consumer-level camcorders. And contemporary ones all record their videos as H264 in MP4 or MOV. The only other candidate is MJPEG in AVI for videos taken with older devices  

If it's legally possible, I'd suggest bundling Avisynth along with your software and coding AVS/AVI import into your tool. There are plugins for Avisynth which handle many other formats. To make it somewhat transparent and simple for users, your menu should contain two entries: 'Open MP4' which handles native import of MP4, as it presumably does right now, and something like 'Open other formats', which when presented with a heterogeneous format, constructs a simple Avisynth script on the fly and executes it, without the knowledge of the user. This way your users don't need to know scripting and your app now accepts many other formats.