It would be great to check constraints on a new or edited statement before it is saved, so that possible errors can be corrected before the incorrect statement is added.
(I think this feature has been planned, at least as a “nice to have for the future”, for some time – I remember it came up when the API was added – but as far as I can tell we didn’t have a task for it before.)
On a more technical level, this means adding support for passing JSON snippets (for snaks, statements, of even full entities?) instead of entity or claim IDs into wbcheckconstraints (or a new API module?), which could also be used externally, e. g. before a large data import. (For instance, the HarvestTemplates tool already includes a partial implementation of constraint checks, and aborts imports which fail some constraints.)