What is the soul of Tales of the Valiant?
I like that it's a setting-neutral 5E implementation, without the plethora of product identity gods, monsters, and other Wizards' IP mixed in here. The game feels like a clean-room 5E, ready to be dropped into any setting and start playing.
In that sense, it feels like an OSR game. But ToV is not an OSR game.
What is the soul of the game? It is a feeling and a reason to exist. It can be a place, such as Night City, which is the soul of the Cyberpunk game. It can be like the original D&D setting, Mystara, and the B/X and BECMI rules for that world. But a game's soul is more than a setting; it is a reason to be. GURPS does not have a specific setting, but it does have a soul in that it can be anything, anywhere, and at any place, and you can be anything.
A game's soul can be the system that defines the characters, encapsulates advancement, and establishes the level of heroism they can attain. Tunnels & Trolls is another excellent game, a fantasy game with no specific setting, but it has a definite sense of purpose and belonging. This is the game of strange tunnels under the surface of a savage world, filled with balrogs, trolls, and diabolical traps and puzzles. There is a feeling that unifies the game as a whole.
ToV needs to be more than "not D&D."
This is setting-neutral 5E, but we also have a setting here, Midgard and the Labyrinth, and that can encompass many worlds and places. So any D&D setting can be dropped into unify the rules behind ToV, and you can pull in whatever you want. The adventures and monsters in 2014 D&D will need to be replaced by ToV equivalents, since 2014 encounters will be a step weaker for ToV characters.
Midgard is an ideal home and feels close to the soul and spirit of this game. Midgard and the worlds strung across the Labyrinth form a strong backbone to this game and universe. Perhaps it is best to focus here and rebuild outwards.
There are plenty of great ideas for campaign worlds in the Labyrinth books. Perhaps the soul of the game can be found here, in newly created worlds built only out of your imagination, without legacy product identity IP weighing your thoughts down, and returning the bad guys to the same old beholders, mind flayers, githyanki, and other monsters overdue for the Monster Hall of Fame and retirement dungeon.
If a 2014 D&D adventure has a lot of custom monsters, that will be a harder thing to port over. You can still run these, but you will need to bump encounters up by a CR level to balance them to a good place. 5E is often concerned about balancing encounters, which is a flaw of the system. So many classic settings will need conversion, and many of the 5E mega dungeons will be a more complex conversion. It can be done by replacing and then creating new encounters, adding hit points, and tossing an extra die of damage here and there, but it is a little more work to do it this way.

D&D had a soul. It lost its meaning when modern writers took over and self-inserted themselves everywhere, making the entire game about identity instead of heroism, and rendering gold worthless in some post-capitalist statement. Instead of finding themselves through the game, like ham-fisted writers, they simply wrote themselves as the hero, ignoring the hero's journey, starting everyone off as awesome, and the story becomes a tedious journey to even greater awesomeness.
Tales of the Valiant shares that unfortunate legacy, but the danger is higher, and the balance better. The Player's Guide 2 will help, since the rest of the party archetypes will be filled out, and we will have a better view of the classes and roles of heroes. I need to find the essence of this game to better grasp what it means to be a fan, and to dream and play within this system. In other games, like Dungeon Crawl Classics, the soul is palpable and feelable, and you know the game when you pick up the books.
In ToV, I need to find the game's heart and soul.