Gul'dan was/is a very difficult hero for me to design because theoretically he could do anything. Being perhaps the most powerful mortal warlock in Warcraft's history and we haven't seen him in action in real combat in Warlords of Draenor. So there's lots of room for speculation that what kind of warlock Gul'dan is. Is he a Destruction warlock who uses a lot of destructive fire spells? Demonology warlock who summons demon minions to fight for him? Or is he Affliction warlock who focuses on painful curses? He could be any of those. I however chose the Affliction specialization.
Why Affliction? Well, I could easily see Gul'dan as a Destruction warlock but the problem is that they're very similar to fire mages. So I was afraid that Gul'dan would be too similar to Kael'thas if I tried to make him as a Destruction warlock. I didn't choose Demonology simply because we already got quite a few heroes who can summon strong minions. Plus Gul'dan doesn't look like the type who summons demons like your every other normal warlock does. He's too badass for that. But I also don't see Gul'dan as the real fighter type. He avoids it a lot. So I see Gul'dan more like the type of warlock who weakens his enemies from a safe distance away so that his minions/allies can finish them off. And so I decided that Affliction specialization is the best one for Gul'dan.
The second big challenge was to choose his Trait. I was thinking of doing something with Soul Shards (like many others have) but in the end I decided to go for something else. Demon Circle teleportation? No. Most of the Gul'dan hero concepts I have seen have that as his Trait and I don't know why. It's not very warlock like Trait. Sure, warlocks have that spell. But it doesn't give the warlock vibe, you know what I mean? It's more mage like Trait since they teleport a lot. So I started to think that what warlocks really are about. What is the thing that every warlock has in common? Sacrifice. That's what it is. They need to sacrifice something in order to gain power. So I got this warlock idea who sacrifices his own health to gain mana. His Trait is Life Tap you use to sacrifice your health for mana. That sounded great to me and so I decided to go with that.
However, this Hero Concept is not done. Most of the spells above may be changed in the near future. But you should get the idea what I am aiming for here. If you agree with this Gul'dan warlock hero approach and you got ideas what spells to give him, feel free to give me suggestions.
I did sent you all Gul'Dans (or almost all Gul'Dans) not to show you to do not the next one. I just wanted to help you to find some suggestions for talents/abilities. Also, you wanted to do Gul'Dan not being present in battle (this doesn't mean not fighting or intervening into it in any other way), so if so, these defensive traits like Spell Shield or Regeneration Master don't suit to him. For me. With these perks, it would be like Gul'Dan in the middlest of fights and taking lots of damage. But it's just my opinion, Conjurer's Pursuit would be better than Regeneration Master, but the other way of stacking stats would be even better for him, like +2 damage per second to Curse of Weakness for each takedown or whatever else.
By the way, what do you think about my Gul'Dan? :P
Anyway, I have updated the concept by changing his abilities quite a bit and I'm quite happy with the changes so far. The point is that Gul'dan is about cursing enemies making them weaker, get mana or a shield by sacrificing his own health which he tries to replenish back by draining life from his enemies. I don't think he can be classified as an Assassin because he's not doing great amount of burst damage at any point. He's an Affliction warlock with high sustainability and curses his enemies from afar making it easier for his allies to kill them. I am going to start working on the talents really soon so stay tuned for that!
http://www.heroesfire.com/hots/concept/guldan-1105
http://www.heroesfire.com/hots/concept/guldan-1092
http://www.heroesfire.com/hots/concept/guldan-1005
http://www.heroesfire.com/hots/concept/guldan-wip-970
http://www.heroesfire.com/hots/concept/guldan-dot-focused-complete-857
http://www.heroesfire.com/hots/concept/guldan-quot-imp-quot-ressiv-797
http://www.heroesfire.com/hots/concept/guldan-673
http://www.heroesfire.com/hots/concept/guldan-595
http://www.heroesfire.com/hots/concept/guldan-wip-557
http://www.heroesfire.com/hots/concept/guldan-complete-499
http://www.heroesfire.com/hots/concept/guldan-347
http://www.heroesfire.com/hots/concept/guldan-156
http://www.heroesfire.com/hots/concept/guldan-23
http://www.heroesfire.com/hots/concept/guldan-340
http://www.heroesfire.com/hots/concept/guldan-1176
http://www.heroesfire.com/hots/concept/guldan-1221
HERE BE GUL'DANS
If in need of help or lack of ideas, check other Gul'Dans and they might give you some. The last Gul'Dan is the best (because it's mine)(well, it's a joke, but maybe it has it's potential :P). I think Gul'Dan should be one of these long-ranged heroes and one of these, that don't need talents like Spell Shield, Regeneration Master or Blood for Blood. As a specialist, he should have some summoning, be expert at pushing or bring an unique playstyle (yours is for me like an assassin). I never player World of Warcraft, however I do know the universe a bit, and always when I hear Gul'Dan I summon the moment "What are you looking at, Eye?" and Gul'Dan launching his Chaos Bolt through the whole continent to kill the mentioned Eye. Of course some of these Gul'Dans might be "low", I just copied all I found and pasted here :P