Below is my opinion of mostly minor tweaks to balance the demigods and create diversity. 

I think this is a two step process.  Step 1 is adjusting talents (slight nerfs, buffs and revamps for totally useless stuff).  Step 2 is a complete revamp of itemization.  Despite some minor issues the demigods themselves are fairly balanced, I would say they are 90% of the way there.  Itemization however is more like 10%.

• TB - Very balanced overall, if a little difficult to play or learn.  He has distinctly different play styles that can all be effective.  The only thing I would change on Tb is increasing the duration of Frost Nova 1 by up to half a second, or reducing cast time.  Most people aren't aware that level 1 doesn't actually help you escape because of the hidden animation cooldown.  
• UB - Slightly OP and very bland, he really only has 1 good build. 
  
  • Only UB and LE have > 6.0 base speeds.  UB has both a slowing talent and a speed talent in addition.  He shouldn't have all 3 as passive abilities. I would probably just nerf the base speed to 6.0.  
  • Acclimation is effectively a passive buff late game.  An internal cool down of say 10s so it can only be up 50% of the time would alleviate that.
  • Grasp is just broken.  First, it can be canceled while retaining the stun.  Second, it's stupid to take more than 1 level of it.  Third, a 2s full stun at level 5 is extremely good.  I would remove the ability to cancel it, decrease the hp drained at rank 1 slightly but increase the hp drained at 2 and 3.  Lastly, scale the stun duration, maybe 0.5 at rank1, 1.0 at rank2 and 2.0 at rank 3.
• Regulus - Close to balanced IMO, but extremely easy to play poorly, he's also noob bait which gives him more of a bad rep than he deserves imo.  He's close to having 2 distinct play styles (ability spam and AA) but one of them is ineffective due to current itemization.
  
  • His level 15 abilities all suck except mark.  If anything the random stun should be at the end of the AF tree since it's based on # of attacks and AF usually stacks haste. 
  • He has 2 activated slows (mines@10 and mark) and one passive slow while he has 0 speed increases or stuns.  I would just buff the passive slow to have a second ability (just like UB's slow) and end in an actually good talent (like UB's).    
• Rook - This guy is almost the pinnacle of balance.  His greatest strength is also his greatest weakness. The only problem is that structural transfer after rank 1 is utterly worthless.  No dg should have a whole line of nearly completely worthless talents. 
• DA - His ability spam build is very good, it costs a lot of mana but it's devastating so that's fine.  His problem is that he lacks diversity.  He has all the talents for a cool AA build (like reg) except that the devs forgot that melee characters will take a lot of damage vs ranged characters (like reg) and he has no way of mitigating that which kills AA (itemization kills it as well but I'm assuming itemization is fixed in my little theory land).  All of his AA abilities should have a passive life drain effect that stacks pretty heavily towards the end allowing him to be in melee range.  Right now, literally half his talents are practically worthless.
• Oak - Ya, if there is every a demigod 2 the storyline clearly has oak winning the demigod 1 tournament and becoming a god.  Then he kills some other god(s) because he's bored and now there is a new tournament, hence Demigod 2.  He has good diversity of builds.
  
  • Shield 3 should be 5s, not 6.  Being able to shield lock AND tp with immunity is stupid.  Maybe at level 4 shield.
  • Last stand, while fun, is counter-productive.  That's ok though, it helps noobs learn.
• QoT - Easily the most underpowered DG of the bunch.  She has way too many minion talents to allow for an effective minion build, and her ability build is way too mana intensive, even with hungarling's.  The real problem is that she's easily the most micro management intensive dg, which would be great if we were all Korean, but we are not.
  • Mulch shouldn't require a target.  Auto kill the lowest health one plz.  Also I would make it instant.
  • Shield and mulch should have no animation cooldown interrupting movement
  • The entire uproot tree is worthless, even level 1 can't be situationally good (like rook's structural transfer)
  • One of the minion trees (compost or entourage) should become a passive or aura of some kind.
  • Tribute is a cool idea, but it's essentially worthless for how late in a game it comes.  I would give each rank of entourage a mini-tribute, with the final rank applying to the entire team.  
  • TB can be effective as either fire or ice, without having to constantly change forms.  QoT should be similar in having a minion form and an ability form.  Pro people can come up with a super micro hybrid build on their own but the two best abilities (shield and ground spikes) shouldn't immediately force most players into a 5s stance-dance the rest of the game.
  • Even after all this QoT would need to be reviewed.  So much would change...
• Sedna - Sedna is pretty good as-is.  She has so many good talents that her diversity comes in which ones she gets in what order.  The problem is, she's a general and has literally the worst minion build of all generals which limits her diversity to just an ability build.  QoT's has 4 talent lines devoted to minions (5 if we include moral), Sedna has 1 with a small minion passive tacked onto a really good passive ability line (again, not including moral). 
  
  • Yeti's need to do something really cool besides do meaningless amount of damage and then die.  Something cooler than say, exploding for a lot of damage but only after 5 talents points have been invested in them.
• Erebus - Despite having only 2 small minion talent lines he has a pretty decent minion build.  His ability builds are great. A little too great some might argue.  I think he's pretty close to balanced.  Like UB he has a base speed increase, a slow and a movement increase, but unlike UB his slow and movement increase (swarm) are both activated on cooldowns with significant cost.  I think he's fairly balanced as-is.
  
  • I would delay mist removing all debuffs till a later rank, it's a bit too powerful at level 1.  
• Oculus - Ah my favorite DG.  Right now 4 of his 7 talent lines are bugged.  He has no diversity at all and only 1 good build.  He'd have the worst minion build of all if it wasn't for the fact that after 5 points his minions become really good by virtue of how much they suck (dying fast).  Before that they are nothing special AND they come with the drawback of 'expiring' early unlike shamblers or yetis.  Oak and Erebus have 'free' minions that have proven to be useful enough to justify them sometimes.  All of the activated minions (shamblers, yetis, balls) are never seen, unless the player is a total noob.  All of them need something cooler to justify their cost, especially oc's balls since they have a drawback.
  • I would give the balls a base explosion when they die, that scales up per rank.  Maybe 50, 100, 150, 200 per ball per rank.  Then they jump to 400 at 15 and have one other cool small effect, maybe their zaps start to chain, or the explosion stops enemy movement (not a stun or interrupt, just a movement impair of which this game has 0).  Actually I would give them a mini chain lighting at 15 and give the yetis a movement impair at 15.  Shamblers already explode and heal a ton on an activated ability.
  • It's obvious that sacrifice was modeled after acclimation, yet sacrifice sucks.  Maybe if after being hit for 500 damage ALL damage done to oc for 5s healed allies for 40% it might be worth the points.  Including minions (except balls because that would be counter intuitive).

That's it.  Mostly minor stuff here and there, except QoT.  I'd really like to see every dg have at least 2 viable builds.  To that end, itemization would need to be completely overhauled.  It would be nice if all items had a little health and mana and than they varied based on utility from that point.  It would be nice if there were more than 2 items that increased movement speed, or lowered cooldowns, or increased crit rate, or gave a lifesteal effect. 

If all 5 slots could be filled with items that each gave some hp/mana but say, DA could grab several items that increased lifesteal,  crit and avoidance we could see more AA builds.  Same with reg, only he would want speed, faster attacks and crit.  If they could do that without totally gimping hp/mana because they didn't buy the 3 hp/mana items everyone else does it would certainly increase diversity.  Being able to build on a SoR build rook with further cooldown decrease items and armor/avoidance would be fun.