If the characters were cards, what abilities would you like them to have?
I'd like Davriel to steal cards out of people's hands and then play them. I don't know about Tacenda, though. I've been drawing a blank so far.
What of Davriel had a unique mechanic that read something like; When an opponent casts an instant or sorcery spell, -X loyalty where X is the spell's CMC and exile it with a theft counter (activate any time you could cast an instant), and another that was +2 and return a card exiled with a theft counter to its owner's [hand/graveyard], you may cast a copy of that spell using any coloured mana?
That would be cool, and I would like to see planeswalker cards with odd mechanics. But I think it would be a lot of complicated wording to do something that would, in essence, be very similar to:
Minus loyalty: Look at target player's hand an exile a non-land card from it.
Zero Loyalty: You may play cards exiled with Davriel, and may use black mana as mana of any color to cast those cards.
Maybe Tacenda would have a tap target creatures effect?
That's a pretty good idea.