Hi, Blizzard mentioned that there is a problem with buffing Illldan if you buff him too much, he becomes very strong. Not by himself but due to synergy with some attack speed buffs like (Abathur hat or Stimdron). This is due to his trait it not only heals him. But gives CDR to his abilities. Currently 1 AA reduces all abilities by 1sec. And if Illidan gets some AA speed buff, his ability CDR increases too much, which leads to constant evasion.
My suggestion is to make his trait like this: after basic attacking his ability CDR increased by 100% for 3 seconds (amount of % reduction and amount of seconds is a mater of testing and might be adjusted). This way it does not matter how fast he attacks. Ability CDR remains the same.
This way he will not have an overwhelming synergy with some other heroes. And this opens opportunity to increase his HP or dmg without fear of overbuffing him. Because now he is countered by almost everything, spell dmg, cc, blinds. Maybe make him immune to blinds (he’s blind already 
). Or make some weaker version of his lvl 13 sixth sense baseline. Because you did great with Casia, her armor is general, not just physical as before. And she does not have some niche anti AA role.