So.. What should I actually be doing?

[oshfdk]

Actually, I realized the question was not "what would be cool to find", but "what should one be doing". I think if you have some novel ideas or passionate about some area of investigation, it's much easier to start there. On the other hand, some of the tasks you mentioned appear harder than the others, so here's my perspective on the relative difficulty of the tasks you mentioned:

-Finding voynichese text in another manuscript

I guess this should be fairly hard if you don't have some exclusive access to a trove of undigitized manuscripts, preferably from the XV century Europe. Voynichese is quite easy to recognize, especially the gallows characters, and they do look remarkable, so the fact that no-one has found anything yet means it was not very popular and I'd say likely was created specifically for this MS.

-Finding a text match for a sentence or paragraph in another plain text manuscript (lords prayer, charm, recipe etc)

I'm not even sure how to approach this.

-Finding a 1:1 matching image/diagram in another manuscript

I think this one is more likely than finding Voynichese text. Again, access to a large number of contemporary manuscripts would help.

-Finding proof of ownership in the currently unknown time period

I guess this would require access to some archives?

-Finding a signature/other marginalia match 

I spent some time last year trying to find a match for the You are not allowed to view links. Register or Login to view. squiggle. I think I managed to track down about 50 signatures and 15 ex-libri of various persons of roughly the right time period with roughly matching first or last name. I haven't found a match, but learned a bit about the European history. It was fun.

If you have any leads or clues regarding some part of the marginalia, it could be not very hard to verify or disprove various ideas using existing online resources.

-Creating a reproducable translation method that translates at least full paragraphs into meaningful text (not things like "apply forcefully candle goat king slowly moon") with very few degrees of freedom in how you choose the translated word (would even this be enough?)

This one should be quite hard, given a lot of people tried and failed so far. For starters, I think you should have some model of how Voynichese works, a model that would explain many peculiar features of the text, and you should be able to explain (to yourself in the first place) why this model hasn't been successfully identified and decoded so far. 

On the other hand, this is my personal favorite, so I certainly won't argue that it's pointless, etc... One just has to bear in mind that the success here is quite improbable and all the odds are definitely against any specific person achieving this. On the other hand, I think that the chance of somebody finding the right approach is quite high. So, the more people try, the better the overall chances are.