It sounds like you had a bad matchup. Where were your allies? you talk about synergy among the enemies but complain about allies teaming up. As Frost TB you have many interrupts and debuffs, including a very powerful group stun. With all your gold I can only assume you were very well geared. What of your allies?

I admit Oak/Sedna/QoT is a pretty extreme matchup. Tons of minions and priests, and if they were smart, they all chose 3 different monk types to stack healing better, along with Shield, Heal, and Bramble Shield. But we need to know more about the game you played it. Skill damage certainly does taper off towards the end though with artifacts entering the fray, that's something I can agree with.

Perhaps if you'd invested some of your huge excess gold into your citadel, you might not have run into the issues you've found. It seems like your topci here is a good example of how sometimes the best equipped demigod can't save you if you ignore your citadel upgrades.

A few things that crossed my mind when I read your post:

1. TB does pretty much the same damage as anyone else with autoattack without any enhancements.  The ability to increase his autoattack damage by 50% puts him considerably ahead of everyone but UB with Beastial Wrath active when it comes to potential late game autoattack damage.
2. TB is not the only magic damage character.  Ooze+Spit UB is heavily magic damage based, for example.  If your end goal is to make TB better late game you need to realise a character's effectiveness is completely relative.  Altering a game mechanic which affects every DG won't necessarily accomplish your goal.
3. TB has a 50% attack speed reduction with permafrost and RoI.  Another reason why he shouldn't necessarily fare all that badly with the current balance and why upsetting that balance might not make him more viable.
4. There are items which negatively affect autoattack damage and none for magic damage (excluding the bulwark which affects all damage).
5. In an ultra long game which you seem to have had there's always stormbringer which does increase magic damage indirectly.
6. You imply that spell power improving items wouldn't affect shields and heals.  Why? 

• If heals and shields were affected by these items how would it affect your gameplay?

Okay I preach this all over the place, but once again...TB's influence on a game is more than his ability to kill enemy DGs. He kills enemy creeps better than any other DG that I can think of off-hand. If your enemy has a bunch of giants with 400hp vs your bunch of giants with 2000 hp, then your giants are gonna plow through to their citadel.

People always neglect to think about that when they talk about TB's balance. I think GPG was completely aware of that though.

some sarcasm: I think that it would be great if I could upgrade the AoE on my Torch bearer enough via items to one shot a swarm of giants with my fire nova. In fact, they should fly in the air once I buy enough magic boosters .

It would be quite an interesting match to watch...Your team has huge damage advantage with the builds you mentioned and your opponents had high survivability.

I guess it went down to team work really but that is contradicted by the fact that you outleveled them meaning you were striking quite a few kills. TBH Lord E and Oak are usually late bloomers and they shine mid game so that shift of power may have confused your team.

Minions and creeps should have never been a problem with a TB and Mines Rag in hand but I guess your team work didn't match up to them in the end. If you wanted more kick you should go fire and not blame lack of magic boosing items. (BTW cool down and cast time reducers are there)

The OP is correct.

Why is there no way to increase the damage done by skills?

Similarly, why is there no way to mitigate the damage done by skills?

Why is Sedna's pounce hitting me for 1000 regardless of my armor? Why is TB doing 1050 damage with each fireball?

Here are my suggestions:

The more total mana you have, the more damage/healing your skills and abilities should do. 1% increase in damage for every 100 mana beyond base mana sounds pretty reasonable to me. This would give casters an incentive to stack mana instead of hp.

Also make it so that armor provides some mitigation against skills. 50% physical mitigation should equate to ~20% mitigation in skill damage for instance.

These two changes would make the game a lot more balanced, introduce a LOT more different builds, and would also make it so that certain classes do not become obsolete as the game progresses.

I don't quite think this would make the game more fun... I do think that there should be an ability, like Morale, that boosts ability damage output by a percentage. This would make builds more diverse.

I think TB suffers from this apparent stagnation of spells more than all the other DG's, and that's what has led you to this conclusion, but I suspect the spell damage increases you are suggesting would end up unbalanced.  50% more damage late game might be ok on an ice TB, but you don't really want to get hit with a 2000 damage spit and a 1500 damage pounce do you?

Additionally, you didn't take the spec that would give you the most damage ability.  I can understand wanting to be able to kill a level 12 one on one when you are 5 levels higher, but part of that is speccing for the ability to do so. Full ice is definitely a utility/group build, not enough damage to take someone down.  Mostly Ice with Fire ball and the synergy would have really turned the tables for you I think.  Fireball, nova, rain of ice, deep freeze, fireball is a lot of damage at level 17, and might give you the soloing ability it sounds like you want.

Late game UB in particular benefits more from +armor and +attack damage than TB can in either of his forms. If an UB wants to sacrifice either of these stats to increase the damage of 1 skill, I don't see any reason why this wouldn't be fair. Sedna as well, I find myself late game with lots of survivability but only 1 source of damage, pounce. Increasing spell damage on sedna would potentially let me create more damage output, but since my only skill based damage is pounce I would be sacrificing a lot of HP for a small additional amount of damage.

In the end, + increase to spell damage or spell crit chance will benefit characters that have a large amount of skill based damage, like regulus and TB, without upsetting balance in favor of would be considered 'imbalanced' classes. There is a problem with damage not scaling properly in proportion to HP gained per level and per gold, it should be counterable by allowing damage to be scalled with gold without augmenting auto attack. I still fail to realize how any player of this game could post anything stating otherwise.

Well heals and shields don't scale either.  Wouldn't your problem still persist when those started scaling up too?  You're not going to solo a sedna who can heal for 2400+ who'se getting 2400+ shield as well.

In fact I would say that you would fare worse in that situation than you currently do, even if your RoI did 1200 damage.

The only thing they are overtaken late game by is people who stack HP at the expense of every other stat else. A regulus that does 400dmg per AA is not as problematic as a regulus that does not go down in 4 fireballs.

Don't forget that items cost gold and takes a slot. Spell damage items should ofcourse be balanced if implemented.