I'd say the best way to buff him would be to better define his niche as an anti-bruiser, so I'd buff his ultimate and the AD drain from his Q
Probably something along these lines:
Q: AD reduction is doubled against champions (for a total of 20/25/30/35/40 reduced AD), but make the AD buff and debuff last for 4/5/6/7/8 seconds instead of 8 seconds at all ranks (because giving him an extra 20 AD debuff without some form of compensation nerf would be a bit much).
R: Trundle drains 20/27.5/35% of the target's max health and 40% of their armor and MR, half instantly and half over 4 seconds (just like live) but receives 120% of the stolen stats up from 100% (so he heals for 24/33/42% of the targets max health, and gains a total of 48% of their resists).
Also, if this isn't already part of the functionality, the heal amount does not correlate to the damage dealt (basically, it will heal trundle for exactly the same amount regardless of whether the target has 100 MR or 1000 MR, just for consistency. This may already be part of how it works, but it is unclear from when I play him (and I play him a LOT)).
One other Idea that's been kicked around for a while is to make his pillar's slow a lot stronger near the center of the slow field, while slightly weaker at the edges, something like 20/25/30/35/40% at the edge and 40/50/60/70/80% right next to the pillar, instead of a flat 30/35/40/45/50%.
