When I saw your pokemon on the side pane I immediately thought "Torterra's Mega evolution should really be Grass/Rock." I'd maybe think about giving it an offensive ability though--it could have an ability like Tough Claws but boosting all physical moves (but by a lesser percentage, say 25%), or it could have an ability that would power up its Ground moves--or it could have a Ground-type Pixilate-like ability.
A new defensive ability on the other hand could be something like what Furfrou has, but decreasing what damage Torterra takes on the special side. I'm thinking it'd be called "Element Armor" or "Solid Ground" (somewhat distinguishing it from Solid Rock). This of course could easily get out of hand in the Sandstorm condition, so I maybe wouldn't boost Mega Torterra's special defense at all. 95/109/105/56/75/85 would become 95/146/133/51/112/88 I think, if it indeed got that ability. That is, needless to say, quite tanky, and the speed drop should help it in Trick Room. More or less Torterra would be seeing sizably positive stat changes all around, the special defense "buff" being in effect and the speed change being negligible even absent Trick Room as a) Torterra wouldn't be losing out against any pokemon truly invested in its speed, and b) Torterra could still be blazingly fast after a Rock Polish (although Torterr could now even be a mixed sweeper!).
Such a stat-cheating ability makes it easy to improve Torterra. Another way to give it a stat-cheating ability is to give it something like Huge Power, but for its defense stat. In that case its defense stat would be unchanged and the "special defense buff" would mostly in the fact that its typing was changed and could be raised via Sandstorm (although it would be boosted slightly also). 95/109/105/56/75/85 would become 95/159/105/51/120/95. That is also of course quite tanky, albeit on the physical side. Since we're investing more in less stats, the attack and special attack stats are increased by larger amounts.
I'd also try and sneak a move in Torterra's movepool--this would be a "Flying-Press" Rock/Ground move, primarily typed Rock for use with Torterra's new type. There would have to be a slight rule-tweak for this move, as Fire pokemon shouldn't necessarily be doubly penalized for being weak to both types.