Some form of innate tankiness would help him immensely. He has no sustain or tankiness built into his kit, which puts him at a large disadvantage in pretty much every top lane matchup. Renekton has sustain and a health bonus, Shyvana has free stats, Mundo gets sustain, Trundle has sustain and free stats, Garen gets free stats and sustain through his passive, even Riven has shields on almost no cooldown for trading. Nasus gets Lifesteal and bonus HP, and other champions seen top lately are all ranged and even they have some form of tankiness or sustain (Lulu gets shields and a health increase, Ryze has spell vamp from his ultimate) while Darius is unfortunately left to rot; if he gets poked down, he simply has to leave lane and come back at a bigger disadvantage in gold than when he left.
I'd like to see something along the lines of every stack of bleed out giving him .5 (+0.1 per lvl) HP5. Level 1 and you Q an entire minion wave? 3.6 HP5 regen while the bleed ticks on those minions. You have a 5 stack on the opposing champion during a trade at level 8? 6.5 HP5 while fighting them.Q a cannon minion wave and opponent at level 18? 8 targets at 1 stack give 2.3 HP5 each, so 18.4 HP5 while those bleeds remain. Even that might be a bit underwhelming late game; something along the lines of 1.0 +(0.1 per lvl) might be closer; at level 18 with full bleeds on all 5 opposing champions, you could be at a potential 70 HP5, which would reward excellent positioning and team play while only being a very, very temporary case because of targets dying or bleed stacks falling off.
Not a massive buff by any means, but something that would allow him to sustain himself in lane and increase his innate durability during team fights. This would also contribute to making his early jungle clear a bit more sustained, potentially opening up a new role for him that is only a niche pick at the moment but could become a more permanent alternate role for him as well.