Here's the difference between Blues Brothers and GB.
Blues Brothers, in all its glory (I loved the original), was still a fairly corny flick with silly comedy and fun musical numbers. Plotwise, not much substance. I still loved the original. But the original should've stayed on its own. After that, there's not much more to tell. Bottom line. Unless you want to rehash corny, silly fun, which, sequelwise, is not smart.
Ghostbusters is a film a little richer in plot and a little stronger in story. It had (and still has) a strong potential for continuing stories. That's why RGB was such a strong cartoon. Period.
I have to agree, if done badly, it would be bad for the franchise, but GB3 is still a project that could be respectable.