…wouldn’t it be much better to have fel flame back?
On a purely thematic and “fun” level, a spell with direct damage would be very useful for movement in both pve and pvp. The dynamics of the old spell could be modified, for example giving 3 charge and CD 30 seconds, which gives you a shard on each cast and maybe a shadowflame DOT after the direct hit. These are examples, but the spell can also only do damage without giving resources and have no CD or charges.
Instant corruption is out of theme in the current demonology warlock, among other things it has such insignificant damage that it makes it really useless.
Of course it would be great if fel flame also returned for destruction to help the spell in movement and under pressure in pvp