Reksai needs a Sion/yorick/poppy level rework honestly.
She never really hit the mark of being a shuriman terror monster that immigrated from the void. She always just felt like a cave creature with no personality, and after her gutting nerfs, shes a knockup bot that provides vision. That's it.
My suggestion is definitely give her a different ult - no global utility. Keep her tunnel as the one ability that's always kept in these reworks, and rework everything else. She should feel like a true burrowing void beast that spits void venom and shit. There has to be a reason she chose the desert to inhabit, right? It must be because sand is easy to manipulate and burrow through.
NEW ULT SUGGESTION BECAUSE I'M GENEROUS. THANK ME LATER RIOT.
R - Desert Void
Rek'Sai burrows deep into the ground, becoming untargetable and unable to take action for 3 seconds. During this time, she begins opening a void portal in the rift, creating a massive zone above her that slows enemies by 30/40/50%-99%, increasing over the duration. A portal to the void opens after 3 seconds, fracturing the land above her and forcefully throwing all enemies still inside to the center, dealing 200/350/500 (+1.0 AD) Physical Damage and stunning them for 1.75 seconds.