Please add a button to add a new blueprint parameter variable

[Alfonse215]

Note that with 2.1, value parameters that get orphaned by removing the entities they affected from the blueprint now can stick around, so long as they're being used by other formulas.

So it'd be really nifty if we had a convenient way to *create* such values. You know, without having to add an object to the blueprint, parameterize it, and then delete it.