If memory serves correct, during beta we had the chance to play the training missions with any card in BDex, a mechanic that seems to be missing now.

(Again IIRC, it used to work like this: when given the chance to modify our chosen deck before starting a training mission, we could pick any card in BDEX to replace our current selection). Of course, it was only available in training missions, and not PvP.

That option was great for testing cards we did not yet posses and see how they worked (I remember learning the deck rules for Heroes that way), reviewing opening strategies with which other players would boot our behinds, and/or checking/testing/detecting the occasional bug.

Any chance that mechanic could be brought back again?

If not for every training mission then maybe as, say, an eighth training mission?

Doesn´t even need to have an AI opponent; just a "Test Drive" field (or "Fire Range" field, if we want to stick with military parlance! ) with a few Recons and Helis for target practice would do.