My two cents:
1) Andreas, GO FOR IT! Whether you do a re-write, or an extension, have at it!
2) UI: Whatever changes get made there, the end result should not be worse than the current.
3) UI: Resizable NAVI - Yes! If you go to a single window preview/render solution, then resizing should automagically take care of itself. I see why there were two windows - one to render, and one for the Navi. And with regard to the Navi, I am not overly fond that it doesn't quite show what the render will (even at reduced pixels). I do understand why it works that way, just not fond of it. Don't get me wrong, I'm not displeased either.
4) Formulas: Having some way to swap formulas in/out (and around) without losing a previous configuration (without doing a lot of file saving/reloading of trial parameter sets) would be great. For example, I've got three formulas going in the set, I like the Render/Navi result, but I want to try "this" without it being added to the parameter set unless I like the result. Think "scratchpad." If the Navi could do that... If that makes sense...
5) I would love a GPU render. Processors can't be swapped out like graphics cards can...
6) Formula creation: I'm nowhere near that, but, perhaps some day I could be if I could be taught the math behind it all (and that's a huge challenge).
7) I'm a visually-oriented person, so, anything that makes the interface work a little more visually is a good thing for me!
This is why I like MB3D over most anything else!
Lighting: I don't see why this couldn't be an open-ended thing; in other words, why the 6 light limitation.
8a) Lighting: Spotlights!
8b) Lighting/DoF: Currently the DoF function doesn't play well with the lighting at times (blurs the lighting, especially volumetric) - that really needs fixing.
9) Formulas: As with lighting, I don't see why this needs to be limited to 6 either. But that's really not a big deal, and probably overkill to go beyond that number - but it could happen!
I like the idea of Bokeh and better DoF (maybe add some blur types - Iris, Field, Tilt Shift).
Otherwise, I will take whatever "you all" eventually create (beggars can't be choosers)!
Thanks for taking this on!