Right now it seems like you are on the "infinite damage with Ral" plan as your only way to reliably win the game (possibly assisted by the "double up some giant Jaya's Infernos" plan). I'd definitely think of adding a few other outs:
- Add a few
Time Warp effects with the idea of eventually getting to 10-16 mana and winning by infinitely looping them via
Mystic Retrieval +
Runic Repetition/
Pull from Eternity while you kill them with 'walkers or the slow and steady grind.
- Add
Stuffy Doll,
Truefire Captain, and
Boros Reckoner and similar such cards so you can rockandsock people with boardwipes.
- Add
Call the Skybreaker and
Cenn's Enlistment and some of the "make a token on spellcast" cards alongside
Jeskai Ascendancy and win with a token swarm.
To get room to do some of that (or all of it?), it seems to me off the top of my head that
Mass Diminsh,
Flood in Tears,
Kazuul, Tyrant of the Cliffs,
Firemind's Foresight, and
Ral, Izzet Viceroy are pretty low-impact in your deck. I also think that while they are a ton of fun,
Throes of Chaos,
Mizzix's Mastery,
Past in Flames, and
Ignite the Future don't interact with Sevinne the way you might hope or wish.
- Because Cascade is "when you cast this" as a trigger and the copy is not cast, you don't get to cascade a second time with Sevinne when you retrace Throes of Chaos.
- Because Mastery says "exile your graveyard, and then copy all cards exiled this way", flashing it back with Sevinne does nothing special.
- Giving cards two instances of Flashback doesn't do much, and so Past in Flames is only super great on the first casting so that you can double up on a spell not normally doubleable. That means it will often be a glorified and worse Mission Briefing/Snapcaster Mage/Torrential Gearhulk in your list.
- I could be wrong on this last one, but because Ignite says "if this spell was cast from your graveyard", I believe that if you copy it with Sevinne you won't get double free cards. That isn't bad, as you are still getting the three exiled cards + the three freebies for eight, but it isn't as amazing as you may think.