Heya,
Avid Gator Boots player back in Season 3, somewhat consistently throughout Season 4. I've played him top, mid, and jungle.
He can be a flex pick, but his roles won't ever reach the reign he's had over top (that led to the establishment of "The Renekton Bar".
Current Renekton
Jungle: Thank our lord and savior Dyrus for showing that this is both doable and viable. It's just not recommended because you're giving up one of the best parts of Renekton which is his strong, cancerous early laning. Free single (double if you play your cards right) gap closer, root, nice early dmg, ult still perfect for level 6 dives, can be aggressive in counter jungling or ganks (similar to panth). Renekton doesn't have the late game carry potential to justify taking early kills (secures excluded) so try giving them to laner. Grab the Warrior enchant and maybe a hard dmg item if you're ahead, then beef up super hard. That's right. Jungle Croc is a sustain tank late game.
Mid: Really similar to top in that it's hit or miss (depending on who you're against). He either dominates the enemy or cries from getting kited. Hexdrinker rush and hard AD, maybe one tank item. You're going mid, so your team is expecting mid laner tier damage.
PBE Fury Changes
This is a definite nerf to jungle Renetkon. May knock it out of viability, unless they give the bonus champ fury to neutral monsters as well (or current amount)
For lanes, this is a slight buff on paper, but realistically it's a nerf. The way he worked before was that you build fury on minions, then use that to engage and win trades. But, they nerfed the amount you get from minions and buffed the amount you get from champs. The problem this leaves Renekton with is that he needs fury to win trades, but he has to trade to get that fury. Like needing a car to get to the job that you picked up to have money to buy the car.
Either you're Leroy Jenkins 24/7 or it's gonna be a lot rougher. This is a bigger nerf mid than top because most mid laners can play safe and starve him of meaningful fury gains.
Other Good Flex Picks
Kayle (Top, mid, jung, supp)
Fiora (Top, mid, jung)
Wukong (Top, mid, jung)
Nidalee (AD top, AP mid, supp)
Hope this helps. Let me know if you have more questions.