- you can cast the back face of that card with Jace without paying its mana cost, if you exiled it with Jace's last ability, and
- you can cast the back face of that card without paying its mana cost with Omniscience if the card is in your hand.
And in general, you can cast Tibalt, Cosmic Impostor (the back face of Valki) with Jace's last ability or Omniscience without paying its mana cost.
However, an effect can allow you to cast only spells of certain kinds in a special way. One example is the recent rule change to cascade, where you can cast a card with cascade only if "the resulting spell's converted mana cost is less than" that of the spell with cascade (C.R. 702.84a); see this thread. Another example is Kari Zev's Expertise; see this thread.
Thus, with Kari Zev's Expertise or Bloodbraid Elf's cascade ability, you can't cast the Tibalt face, since that face's converted mana cost is too high in either case.
EDIT (Mar. 29): Clarification.
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Lets not forget Riku of Two Reflections who can copy this, giving you a total of 11 Doubling Seasons, or 3 Doubling Seasons and 8 copies of anything on the board.
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Artifact - Equipment
Equipped creature has deathtouch.
Equip 1
Next:
Omnath, Locus of the Roil
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Mirror's Envy 3UU
Instant [M]
Create a token that's a copy of target permanent, except it isn't legendary.
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I suggest someone submit a very Canadian planeswalker into the survey for this reason.
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If they were to remove the Reserved list, it would be better if the cards had updated art and borders. This would leave the older cards to retain some value, even with the reprint. Again, IF.
On topic, I hope that they have some interesting things planned for Magic's 25th year.
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