First let us start of with Demonic Gateway
I understand why the game designers left it in its current form as there is a very fine line separating the spell from being too powerful versus nigh worthless. At its core the idea behind the spell is sound, but it suffers from a few problems that render it only useful in specific environments. So I propose the following changes...
1) Remove the tether...
To make it act more like Demonic Circle. There is nothing more frustrating than being confined to a certain area in order to actually use the spell. In PvE and Arena environments this is not such a big deal due to the size of the playing field. Where the tethering issue becomes apparent is in Battlegrounds. I simply cannot go from one end of the map to another without the portal de-spawning.
2) Reduce the initial charge time...
I feel like the 5 second cast time is perfectly acceptable. The problem is the 10~ second delay after the cast goes off and the portal is actually useable. As soon as the portal lands the first charge should be able to be used. This change would make the spell more useable, especially in the fast paced nature of Arena.
3) Allow flag carries to use the blasted thing...
Seriously, this would not be unbalanced at all. A 15 second debuff is applied to whomever uses the portal the FC would not be able to abuse a portal chain set up by multiple locks.
Let us move on to the talent choices Warlock's have once they reach the level cap.
I can understand why this spell's effectiveness was reduced due to players abusing the mechanic in PvE. But I am pretty certain my proposed changes would not upset balance.
1) Increase the passive to 10%.
5% is just too little to be noticeable. Pretty simple.
2) Keep the on-use damage, but now affects everyone who attacks the Warlock and is useable while stunned.
It doesn't make sense for the passive to affect all targets that damage the Warlock, but the on use only affects one target. Gives Warlocks a formidable defense against target swaps in Arena and makes enemy targets think twice before unloading all of their crazy burst we know exists in MoP.
Currently the negatives far outweigh the positives with this spell. The on-use effect is simply too weak to overcome the passive's shortcomings.
1) Change the passive to allow three spells to be cast while moving, with 5-10 seconds refresh rate per charge, a la conflagrate charges. Either remove the penalty or make it 25% increase cast time.
2) On use effect would instantly grant 3 charges and for the duration of the effect spells do not have an increased cast time, or perhaps an increased movement speed while casting.
This one was perhaps the hardest to come up with meaningful changes for. The only practical use for this spell is is in PvP with destruction using it to increase the chance of Aftermath stuns. Other than that the spell has no benefits. So I propose a whole new, on use, addition to the spell. The general theme behind these was the "warlock exploding with fel-energy causing damage to everyone nearby."
Destruction - Next Chaos Bolt will deal splash damage to all nearby targets. Perhaps to the order of 20% of the bolt's single target damage.
Affliction - Next Seed of Corruption will instantly detonate and will apply a seed to all nearby targets.
Demonology - Your next Hellfire cast will ignite targets on fire. Causing them to each create smaller and weaker hellfire versions, dealing damage to all nearby enemies.
I urge everyone to comment and refine these ideas in hopes that Warlock's can gain more interesting and game-changing abilities and talents.