Yeah you are right. If we do add mobility, then the only mobility to actually add tonhim is +5 for his movement speed. They nerfed his bleeds ratio from 200% bonus ad to 150%. If they really want to buff his ratio, I think they can just make his bleed stack up to 7-10 stacks. That way his ukt can deal more damage or you can reduce his damage a bit to make sure he doesn't become 
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Probably be like this: 5 stacks = 100% extra damage on Ult. Any additional stacks will increase damage output by 10% and in total 50%. So instead his ult will deal 150% maximum damage out put. 340 base 1 stack ult. 680 base 5 stack ult. 850 base 10 stack ult. Ratio will remain unchanged because 150% bonus ad (when you have 5 stacks) is quite enough. You can increase but I think 150% is fine.
His bleed can have the same treatment.
Bleed deals 180 (150% bonus ad) 5 stacks. Over 5 seconds.
If you reach 6 stacks then the damage out put of your bleed increases by 10% to the maximum of 50%.
Bleed deals 270 (225% bonus ad) ten stacks over 5 seconds.
You can add more AD for each reset he gets like 10% AD to a maximum of 50% AD. It lasts for how long he can Dunk you but this might be hard to do so when he can completely reset... So I don't know. His ratios = to this: 105% bonus Ad+ 150% bonus ad and +150% bonus ad so that is 455% bonus ad and 200% AD (From his W). His ratio seems quite a lot.
The only way I can think of for adding more ratio is tobmake his pull deal damage equal to Khazixs leap. Or a bit less than that and make it add a bleed stack. That way when he E->Aa->W->Aa->Q and 5 stacks right there. Its kinda hard to buff him in such a way that you won't find any QQ from somewhere else.
TL;DR actually just read tldr. The proposed change in the beginning is just making Darius a time ticking bomb which we don't want cuz fights dont last for too long. A way to buff him is to probably remove the barrier on his Q (deals 50% extra damage from any range it can be like this: once he maxes his Q to level 5, it unlocks and the barrier removes meaning he can deal 50% extra damage without any conditions). Actually if you reset on a kill, then you gain 10% AD for 20 seconds to a maximum of 50%. this could make him abit stronger but hey, this is what I can come up with.
SIDE POINT: Usually a Sivir's ult is really luxurious for a Darius. Having Shureliyas is good as well. Usually getting MS Quints is a really solid choice. If you can somehow get 3 drags and alacrity , then you'll be golden. I feel like Tri force is a really good item. It gives you pretty much you would want, Ad sheen process, MS, Attack speed. Lucky crits and the passive Rage. I feel BC is I won't say better but I'd prefer it. Because of the 20% CDR health and 30% armor shred. Tri force gives you mana and as Darius, any man's won't be wasted.
My runes are usually like this: 9 marks of AD. 9 seals of scaling armor. 9 scaling MR. 2 quints of armor and 1 ad quint. 10.5 AD/ 27 Armor @ Lv18/27 MR @Lv18/ and 8.5 Armor. With this, you can suffice with just Randuins, you have around 200 def with just a raduins. With Maw+Merc+Banshees, you'll have around 200 MR and your team will build a Locket so that is 220+. You can build up to three damage if you want. Here is my usual build.
and the last item is up to you. You want damage? 
. You want tankyness?
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In game with these 5 items you'll have around 230 AD/ 200 Def/ 200 MR/ 3.5k health. Only trade off is the extra movement speed but hey you can get alacrity doe.
while other champions don't have that luxury.