I actually think there's a lot of potential to return Fel Flame at this point now that Life Tap (and Warlock mana management in general) are no longer a thing. I'd like to see a version of Fel Flame that costs a significant chunk of mana, turning mana into Destruction's "movement" resource and making it relevant again. Pinging in the odd Fel Flame wouldn't be a big deal, but sustained periods of movement would run your mana pool dry and you'd have to wait to rebuild it before using Fel Flames again. This would give Destruction something to press while moving while simultaneously keeping positioning relevant by rewarding players for moving in staggered increments rather than large bursts.
Obviously you'd also remove or significantly reduce the mana costs of all the other spells so that you couldn't lock yourself out of using other abilities. You could even do some fun things with mana regen like having it recharge from Chaos Bolt or RoF casts, creating an interesting build/spend cycle for your mobility.