Imminent Doom
For Standard I don’t see issues with him as long as he pings for 1 damage and only pings the opponent.
He is nowhere near as close to be good as Imminent Doom that being a support that also increasingly dealt more damage were the key factors of the card. Creatures are known for not lasting very long in this game. Plus ID was super good in creatureless builds and dealing with those pesky huge HP opponents a la Azor (I am not waiting to cast 400+ spells to kill my opponent, infinite or not).
The one upside I see here is that he is not necessarily bound to spells - I actually see him as more of a Combustible Gearhulk than Imminent Doom (and Gearhulk dealt a TON of damage AND also drew cards). Sure, it would seem obvious that you want to combine him with many low cost spells to draw cards, and probably you can stick the Immortal Sun in there, but again, the palette in the current Standard is nowhere near as scary.
Now if he is allowed to target creatures only, that could get vaguely interesting even though at 1 damage per ping I would rather combine it with enrage creatures (and I fear they will rotate IXN with GRN entering Standard) for the juicy triggers. But even that looks super gimmicky.
So… yeah, if it’s 1 damage he’s going to the bin due to the immense number of triggers required to do anything efficient. The more that damage upscales (2, 3 etc.) the better he naturally gets.
Baral
Spells replacing themselves is what make Baral good, but once again, when I’m looking at Baral I see three huge advantages:
- Very low cost - you can combo into him with Blue Sun Zenith and small mana gain spells - therefore keeps your curve low
- Card draw on spell cast - superb in combo decks that keep cascading into spells
- Generates tons of mana at the beginning of your turn if you have lots of spells in your deck
Niv Mizzet fails on all of those except for the card draw if he will be implemented properly (I would personally see him as an 8/8 dragon that costs 18 or something):
- Upping your curve is not where you want to be in combo decks - there is a reason why Combustible Gearhulk saw little play, and the 25 cost was very tough even for my old Bolas combo deck (achievable though problematic)
- Does not generate any mana value, he’s only a mana sink
- Card draw on its own is combo-ish but weaker than Rashmi
Naturally when you combine the two sides you generate an interesting walker, but once again, this is not paper magic where card draw is much much more powerful and can get out of hand much easier. Why? Because of the limited hand size naturally.
Legacy
As Gearhulk + Baral sees little play in legacy I am not even remotely concerned about Niv Mizzet being a bad combination of the two.
Sure, the potential of Kiora gets opened because Niv is blue (when compared to Gearhulk + Baral that could never be played in monoblue decks), but even so, legacy can already do much nastier things with Kiora to begin with (why bother ramping and drawing with Niv when you can use Prism Array & Part the Waterveil for a much more potent combo?).
Can you go infinite? Show me a deck that can do that with Niv CONSISTENTLY (let’s assume he deals 1 damage at a time). There are multiple questions here:
- Is it efficient enough? Or do you have to wait gazillions of turns to slowly ping your opponent? (think 132 HP Bolas here or 500+ HP Azor)
- How difficult is it to go infinite? And are your cards good enough to hold the bay while you are trying to assemble the combo?
- Is Niv good in a vacuum too (like Baral is) or would it be a strictly absurd legacy combo card?
The one place where I saw people play it was in Ob so they could pair it up with the ultimate and benefit from the extra draws.