Firstly I am happy, with foreknowledge to include custom things in games

Some comments from my experience with multiple big gribblies in fantasy (both doing as I say, and well...not doing what I say, want or would rather they didnt)
140pts is pretty cheap. Most big things are expensive, example Bretonnian hippogryph is 200pts, Orc Wyvern is a bucket load, dragons are multi-hundreds ect.
Its statline has, to me some issues. For starters, WS 6 is way over done, i would say 2-4. WS 6 is a Bretonnian Lord, someone who has been trained from birth to swing his sword/mace around in astonishingly difficult and years to master performances. WS3 is the average human soldier who has had decent training (think empire swordsman) and know how to handle themselves and their weapon. A bear is just swinging its claw studded paws at things and making loud noises.
Bret hippos are WS 4 and they are intelligent, I would make the bear WS 3.
I 7 is omg huge, thats like elves, Skaven and magical item boosted people. I 2-3 more likely. Its covered in armour, has a heavy dude on its back, and its primary method of fighting involves standing up and biting.
I would remove the loner rule, there is nothing in the dwarf codex that would give the rider/mount a look out sir roll, as there are no unit options for cav/monstrous cav.
I would possible ban it from joining slayer units (lol) and gunpowder squads as the noise could frighten it despite training
It should have gromril armour (still only counts as barding for the rider) and armour piercing, metal gauntlets + bear strength = ouch.
Cause fear, thunderstomp (win)
I'm thinking low movement, 4-5, but a bonus to charge and flee as those buggers can really get moving when they need to, but not consistently.
Fire attacks cause fear against the bear (and rider as the bear bucks)
The lords leadership range is increased as if he were mounted on a monstrous mount on a big base (ala an orc warlord on wyvern has 18inch range)
Should be a lord and runelord only thing, as most heroes dont have access to big mounts. And I cant see it fitting with slayers. Not only cos they would like to fight it, but just does not sit right with their fluff.
Points wise I would be looking at 250 at least.
It is damn painful (3+d6 S5 armour piercing attacks on top of the crazy shizzle a Dwarf Lord can dish out, yeah speaking from experience) and although It can be singled out by shooting, Dwarfs can be stupidly unkillable. RO re-roll armour save, RO strength greater then 5 counts as 5 and 4+ ward for one amusing combo.
Dont forget RO 2+ ward vs shooting.
Or RO strength greater then 5 counts as 5 + RO Toughness +1 for giggles. Cannons wound on 5+ oO