I'm pretty sure sion's move speed is hard capped to a max of 345 during his passive, which explains why he can't catch up to people, even when he has more move speed. There are SO many numbers of things that can be done for sion passive, but allowing him to move at the speed that is listed on his stats would be a start.
Other options include:
1- making his damage scale with HIS max health rather than his enemies.
2-giving him something better than 200% life steal seeing as he doesn't build much ad with those few exceptions being very LOW in ad.
3-Give him some bonus tenacity (maybe 15%?).
4-Give him immunity to slows.
5-Give him scaling AD that applies only in passive form to make his autos do actual DAMAGE to squishes and make the lifesteal useful (say 20-90 AD).
6-Give him a slow on his autos.
7-Make his health drain not apply while hard ccd or in stasis.
8-make him immune to grievous wounds.
9-make him deal aoe damage on attack. (Not recommended)
10-give him a suppression if he autos the same target X times(where 2>= X <= 4) (interesting, but not recommended).
11-Give him damage immunity from sources other than his own bleeding. (Not to be combined with cc immunity/ reduction).
12-Make his passive reduce targets armour by the greater of 10 or 10% (potentially scaling from 5 to 20 or something) with every hit up to 3 times.
13-make his speed boost more potent and give it a longer duration.
14-Increase bleed effect, make immune cc of all types.(not recommended)
15-give him a slowing aura (not recommended).
16-Give him scaling move speed towards targets.
17- Make all spells used and damage done to him increase his move speed proportional to the damage dealt.
18- give him move speed on attack.
19-Make his attack animation not cancel movement commands (what I mean is if he autos, he doesn't stop moving to do so).
AND 20-Make him deal a lot of burst damage in one auto which gets reduced every subsequent attack.
These are some of my ideas (and a few popular opinions) of what could be done to make sion passive a bit better. If I were riot I would focus on the ones that make it better late game/ the ones that can scale, because that is when it tends to suck the most.