After reading this, I've done additional testing and editted some stuff.
Thirsting Blade is indeed better. Thanks for the eye opener, def the superior choice.
I will agree, the lvl13 talents are situational, but for the most part, Sixth Sense works in most team comps the best, I find. But like you said, it really just depends.

I definitely do disagree with you on Fiery Brand though. I've played a **** ton of matches as Illidan post-dehaka patch and I've never had a problem with getting my 4 auto attacks in on a target. I mean, either you're alone chasing someone down by yourself or with teammates, allowing you to use your basic attacks consistently till they die. Just keep up with them using Dive and Sweeping Strike. If you're in a team fight, a lot of the pressure will be taken off you if your tank is great and knows how to do his job. I've never really had a problem with Fiery Brand. I mean you said it yourself, most of your damage comes from the basic auto attacks. Grabbing Blades of Azzinoth isn't a bad choice, but its definitely got less uses, since Fiery Brand is a passive and occurs while auto attacking. It speeds up the clearing of siege camps, and most of the time, keeps the pressure on the target since they can't really get away from Illy, allowing him to just auto attack them to death.

Bolt of the storm was editted and explained. Thought I was a bit more clear with it, but I guess I forgot to add it in. It's a situational talent and it definitely has come in handy. It makes Illy's mobility extremely difficult to control, and helps you either get out of sticky situations or secure kills.

And diving your teammates is not always an option. Sometimes they back out earlier than you, sometimes they're out of range, and sometimes, they just aren't there for you to rely on as an escape route. I do agree Friend or Foe does help a ton with escaping, but Bolt of the Storm just adds that extra bit of mobility and mobility & sustain is what make Illidan such a dangerous threat.

Thanks for the post.

No. Being Illidan's damage or most of it comes from his basic auto attacks, Marked for Death just doesn't offer as much uses, even though the crit damage is a lot higher than the other lvl16 talents.
You're attacking a lot faster with auto attacks, and that grants more Fiery Brand damage, or Blades of Azzinoth damage. Marked for Death goes on CD and you're only able to use it every few seconds, which is time wasted where you could be auto attacking someone consistently and dealing bonus damage consistently as well. The other 2 talents basically, to sum it up, have more uses add more pressure.

With Unending Hatred Illidan's job is to kill 200 minions, 20 heroes or a combination of these. Seems like the short-bus, hero-specific, nerfable version of seasoned-marksman. Otherwise, why not just add that to his T1 talents instead of making this new talent which gives a cap of 3 less damage for the same number of kills? Why make the +10 damage some sort of prize instead of a simple .15 per minion and 1.5 per hero?

Can't hurt to check it out but it seems like the only synergistic talents are the 20% attack speed or damage at level 20. It's 37.5 DPS and 45 DPS for basic attacks with either of the level 20 talent boosts, unfortunately, Illidan's abilities and talents are not based on auto/basic-attack damage (even Fiery Brand).

Added explanation of T1 Unending Hatred.

Yeah I realized a little too late, and didn't edit/update that part. Thanks!