I think it would be really cool to have the ability to give Sparky access to one of my decks for training. The basic ones Sparky chooses among get old fast, and she very quickly becomes obsolete. The only point of trying out my new decks against Sparky at this point is as a sanity check that I didn't go too off the rails with a new idea.

However, if, for example, I could have given Sparky my Dimir rogues deck during Zendikar Rising, she would have become a very useful training partner. I could try a new anti-Rogues deck idea, like Kroxa-based Rakdos, against her many times to see what did and did not work for the new deck rather than having to slog through lots of freeplay games until I'd fought enough people playing Rogues.

I know Sparky's AI isn't currently the best (she often targets her own creatures with elimination spells) and I have no idea how she'd manage with multi-mana decks, but I think there's a lot of untapped potential in the Sparky feature and having a decent AI MtG player would be super impressive and might help to retain players who are frustrated with the PvP.