Phew! This is one interesting card! It can either be overpowered, or headed to a long cold slumber.
I don’t think there’s really an in-between with this card. It’s so polarizing, which makes it extremely difficult to evaluate. It’s certainly not a card I would cast often and just not in my wheelhouse.
On one hand, I read this card as simply “gain an extra combat step” for the highly aggressive decks I enjoy playing. These types of effects have not been printed in this color before, and certainly not attached to a low converted mana cost. The last spell attempting this type of effect to any success was Time Stop. There’s been a few imitations along the way such as Sundial of the Infinite. Those likely pale in comparison to Glorious End as a real contender in Constructed play.
I would like to test this in the method I mentioned before, and see if it has any merit in providing the one extra attack that would be needed to win in aggressive strategies. The “You lose the game” clause wouldn’t be much of a factor in that instance. I feel Wild Nacatl, Tarmogoyf, and Kird Ape would be able to seal the deal if provided an extra turn ahead of an opponent.
It really all depends on what players like to do as the metagame shifts. If players are content holding back mana for Magma Spray, and Fatal Push this likely won’t help. On the other hand, if the format slows down where tapping out for a Verdurous Gearhulk is acceptable – then this card can be seriously considered to end a game. Also consider the interaction with Gideon of the Trials emblem.
Whichever the case may be, I do feel there will be an necessary value attached to this card just to hedge against something insane happening due to this card. It’s not a very good space to be, if players looking to acquire this card are deterred by it’s value early on. The one factor I believe to be working in players favor – is it will likely take some time to figure out this card. In the meantime, this could lead to a more lucrative value for players that want to acquire this card and enjoy.