I'm gonna start you off with a hard one.
Is there anything I can do to make bull a useful member of the team as a 2-hander with minimal input from the player? Would doing so provide any benefit or synergy with a rogue?
ohhhhhhh........good question points.
I have actually had this problem on my 1st play thru and didnt figure this out till i started my 2nd play thru which im still on.
As a result i built bull as the following.
Core Abilities
1. Block N Slash- Damage is So-So, but the important thing is the guard generation 15% cause without guard your pretty much just a rogue and thats not good in this case. With the upgrade. Did I mention is has no CD. Sadly this is an overlooked skill. 
2. Challenge- 10% more guard nice, taunts a single enemy if the target is melee then its just a loop of the enemy just attacking bull then getting knockdown via block n slash. With passive bull can do this indefinitely to a single target while your tank holds the rest. 16 sec cd
3. Charging Bull- Another 10% guard gain. He'll mainly this skill just to be brute to opponents whenever its up just utterly be a menace. Even if his target is a mage or archer if it can be knocked down Bull's go it on lock and since most enemy mages/archers run when harassed my melee. The passive is not needed til up get more stamina consuming moves on your him for next stamina move is free. 8 sec CD!!!
These first 3 are a must with these alone he has 35% guard gained!
4. Pummel Strike - Just an all around good talent with nice dmg even if the enemy is immune to stun effect. Also no long windup or recover time like Mighty Blow.
5-8. Just put in his Reaver tree skill after skyhold and hes good to go.
This setup has keep my bull save and allows him to hold his own most scenarios exceptions are dragons, bosses.
Also i changed his weapon he uses from 2hander sword/axes to a Mauls because it swings faster and better Single target.
This way he has guard so he has a cushion when he takes hits and uses reaver skills and the flow of combat if you make a high criting weapon he will always be ready to attack. This setup is better for an AI not actually player may find this boring.
If you get extra points go into the Battlemaster for Combat roll and take off Pummel Strike. Due to his reaver skills putting out better damage.
Also in tactics menu put Bull on Follow and Iron Bull. In my experience he seem to go for the closest target who is usually melee.
Hope this helps.
If anyone has any suggestions/tips or variations please add.