If you were to give these Champions a new Passive, what would you give them?

rek'sai: within coming of a 3000 range of yasuo. yasuo instantly dies and you achieve double kill gold.

Rek'sai passive is not that easy to replace really. Because if we are JUST gonna take away her heal per rage then there is very little strength to reallocate. Xerath is pretty much the same.

Both new passives should be as useful and strong as the old ones.

Maybe a new cool xerath passive would be After spending 500/400/300(levels 1/8/12) mana the next spell is free to cast and has 10% improved damage to champions. Or something like that

I suppose for reksai I would move her fury passive to her W, burrow and unburrow, and then I would make her new passive something like "10% more damage when attacking after unburrowing, spotting an enemy in the fog of war, or hitting prey seeker for 3/5/7 seconds" And for Xerath, I think his passive is fine really, I dont think it should be changed, perhaps make it "for every kill restore X mana"

Xerath - Arcanist Pulse, Xerath leaks magic radiation dealing damage around him in a pulsing circle. Using abilities gives him stacks, the more stacks, the faster the pulses occur and the bigger they are, stacks fade every 5 seconds.

Rek'Sai - Queen of the Xer'Sai, as Rek'Sai builds up fury she begins to spawn Xer'Sai, up to a cap. Xer'Sai latch onto targets she bites with E, and frenzy when she emerges from the ground dealing additional damage over time, she can consume a Xer'Sai when casting her (burrowed)Q for it to bypass minions.

For Xerath I'd have to imagine something like the gathering storm rune. Would fit his theme, Xerath gaining freedom slowly, maybe losing bits of his sarcophagus and gaining ap at some levels.

depends. am i supposed to give them a passive based on what i think fits the champs, or what riot would most likely do?

Xerath: Instead of autoing to get some mana back he absords mana and energy expelled by enemy champs to get mana. Fitting in with his energy being theme, he takes part of what they use to rechange himself. You can run him dry by not using abilities but due to it's range he would need to be closer than usualy to get it's benefits.

Rek'Sai: Just add a HP scaling to it. 190 at level 18 when she could have 3000 HP is only just 6% and she's usually out of combat, while Mao'Kai absorbs 13% or more while in the middle of combat, that's 400 with an auto and again soon after depending on how many hit's he's taken.

Both of those don't need new passives, tbh.

Xerath's passive is there to make him interact with his opposing laner, instead of spamming out spells from 1000 range away. As such, it cannot be replaced except by a similar mechanic to force closer-ranged interaction.

Rek'Sai's passive serves her kit in an important function as a sink for her rage. Without it, she'd pretty much constantly be at full rage and thus do true damage with E.

Rek'sai: [Ambush Predator] Enters stealth while submerged. When Rek'sai leaves enemy vision, gains an Ambush Predator buff that lasts for 3 seconds after being revealed. The first basic attack on a monster or champion consumes Ambush Predator, dealing bonus magic damage and inflicting Fear for 1.2 seconds. This can only trigger once every 10 seconds on a given champion.

Xerath: [Helix Conduit] While Xerath is stationary, he gains 2-5-10 AP per second, up to a cap of 20/30/50 AP. While moving, he loses the bonus AP at half this rate. Reverts to 0 bonus AP on ability cast or when Xerath is disabled. (Each charge of his ultimate counts as a single cast.)