Hello,
My Dream scenario is that the VE understands that the data is pulled from Wikidata and shows a dialog that is similar to the current template parameters. I see the old mayor's name in that dialog, I write the new mayor's name, and the new value is stored in Wikidata. Of course, it must be taken into account that the name is likely not just a string, but a label of the Wikidata item.
The problem I see here is that we mostly do not want to replace a value but rather add another preferred one and state that the old one is now deprecated. This means, we also want to have the old major on the Wikidata item but the new major should be marked as preferred. I think we have to think a lot how this can be implemented best in Visual Editor. Maybe we should just add the value entered there just as another preferred value but this is rather a guess because the data might also be simply wrong and should be replaced. We thus have to investigate how often the value should be replaced and how often the old value should be kept as well. Then we can decide a way how the visual editor should use this data without being to complicated but also offering all options to create the data as intended.
My acceptable-but-suboptimal scenario is taking the user to https://www.wikidata.org/wiki/Q6850543 . This is probably a useful workflow for the tech-savvy editors, but it's suboptimal for a casual editor. I'll go as far as saying that for a casual editor it may be (relatively) more comfortable to edit parameters in a MediaWiki template ("|mayor =[[Milan Ftáčnik]]") than to go to https://www.wikidata.org/wiki/Q6850543 and find the value.
We might take the user to the item's page on Wikidata if he wants to make more enhanced edits like replacing an old edit.
Does anybody have any more ideas about it?
For the technical implementation, we have to find a way which data comes from which item and which property. Maybe we can add some data-item and data-property attributes to the HTML tag in which the data is stored so that the visual editor can easy find these transclusions. However, this might also be a security issue if the attributes are added to elements not containing Wikidata data.
All in all, you are surely not late and this discussion has just started at Wikimania (although it is planned much longer to have something like that). Therefore, I think this feature hasn't hightes priority at the moment and needs lots of cooperation with the Visual Editor team. Perhaps we will start developing not before 2016. (Only my personal guess. :-)
Best regards, Bene