I have always been a fan of Commit // Memory in any Izzet/draw decks, because of its versatility.
Commit // Memory seems like a worse card on both turns than all of our other options. The only real plus side is that it's like two cards in one. The first part is very over priced just to delay a card and the aftermath costs three more than we want it to.
Chasm Skulker helps you deal with creatures. It's so efficient at it, get big then give chump blockers. But anyway, you alone know if your meta needs it.
As for Chasm Skulker, it seems slow for a not great effect. Instead of creating chump blockers, why not run board sweepers instead? I even think Propaganda is better than it. Like you said though, depends on your meta.
Consecrated Sphinx is a good one. I flip between it and Azami in the deck. The Sphinx draws more cards over the course of a game but Azami fuels the Paradox turn better than the Sphinx could.
Lastly, Consecrated Sphinx vs. Azami, Lady of Scrolls. This one is pretty close. As I will be talking about below, I am going to be moving away from the combo and cutting Paradox Engine. I've been playing with it for a little bit and it feels a little slow. I have always liked Consecrated Sphinx, it's a very strong card that requires an answer. Also, we have a lot colorless rocks that can be utilized in paying Consecrated Sphinx's 4 colorless.
I haven't played commander in a while but then I got back into it last night. I played in a pod with three other combo players. While this may not be the ordinary makeup of a pod, I do think that combo is the most popular competitive EDH deck.
I have always looked to make my Niv-Mizzet, Parun deck as competitive as possible and I always knew that Niv wasn't nearly as strong as other combo decks that had green or black in them, but I always managed to win games despite that. I think it's finally time for me to move away from Rowan's original list. I think it's going to be more of a control deck that focuses more on using Niv-Mizzet, Parun to gain board control and card advantage. I don't know how it will work out, but I will try to keep you guys updated and if anyone has any comments/suggestions/questions please respond and let's talk.
A couple of big changes I'm probably going to make:
When I'm at the table with other combo players; they can usually get to their combo faster, empty their hand faster, then they either win or their out of the game for a couple turns. Cards like Wheel of Fortune helped the other people at the table much more than it helped me. Instead of wheels, I'm going to running more hand fattening cards.
Almost every competitive deck runs tutors, Birthing Pod-like effects, graveyard shenanigans, and non-basic lands. I'm going to start running Stranglehold, Grafdigger's Cage and Blood Moon.
Tutors/Extra Turn Effects/Recursion
Since I'm moving away from full combo and I'm adding anti-synergies with cards like Stranglehold, I don't need tutors/extra turns/recursion as much as I did before. I am going to replace these with removal, counters, or hand fattening cards.
To try and stop others from winning before I do, since I will be going down a slower route, I am going to have like 30 cards of removal, artifact removal, Counterspells, and various other disruption. I am going to bring in a lot of "free" spells (such as Force of Will, Misdirection, Pact of Negation, Snapback and Submerge) for disruption and protection so I can tap out and still be able to have a response to someone trying to win or to protect my board. They will also work more with Niv-Mizzet, Parun so I can tap out to cast him and then protect him with the "free" spells. Might try out As Foretold too, especially since it's CMC, I can cast Niv as long as it has six counters no matter how much tax is on him. Seems too slow though.
All these changes are pretty speculative that I really don't know how it will work out. Like I said; if anyone has any comments/suggestions/questions please respond and let's talk.