Make his wall able to block skillshoots. This will make him viable against poke compositions.
It will also add a bit realism and utility. it´s better to add a bit of utility than buffing numbers that can lead to a huge power change.

Skillshoots like Dark binding, Javelin Toss, Mystic Shot, Electro harpoon, Bandage Toss, Enchanted Crystal Arrow, Rocket Grab, Sear , 90 Caliber Net , Missile Barrage, Infected Cleaver, Chum the Waters, Thundering Shuriken, Ethereal Chains, Light Binding, Sonic Wave , Dredge Line , Glacial Prison , Acid Hunter , Bouncing Bomb .

The idea is that the wall will have a hitbox that work the same way as a players hitbox.
Skills like Light Binding only loses the first hit.
Skillshoots with splash will deal damage around the point where it hit the wall.
Skills like Sonic Wave and Bandage Toss should pull you to the wall.

I believe this would fit the troll perfectly. It will make the game more exciting and fun to watch.
Saving your teammates feels great and rewarding. Right now his wall doesn´t feal right, because a lot of heroes have flash, dashes and movement steroids.

huh
interesting Idea
would it block things like Tristana's Rocket Jump or MF's Make it Rain

Trundle IS viable.

I think his wall would be too strong and unfun for the opponenet if it could block the rocket jump and make it rain.
It doesn't look right in the eyes of a new player. The pillar may have to shrink in height so tristana can jump over it.

Then they'd have to give equal treatment to all other walls.
Jarvan would surround his team and make sure anyone trapped inside couldn't shoot outside.
Anivia creates a wall the length of a lane, blocking off any skill-reliant champion's damage...
Yeah...
Trundle IS viable already, he's a debuffer who shreds bruisers/tanks. He's just not very well-received in the current meta which is all about Black Cleaver and AD caster mids, and health stacking (though health stacking is a bit helpful for him, each time someone dies he heals a **** load).