Suggestion: Create another layer of power fantasy with a permanent progression of swappable character stat bonuses, including resistances, attributes, and permanent increases to sockets and skills from items, derived from the current static boss rewards
The feeling of a character progression is an important matter for any good ARPG. The current implementation for getting passive permanent bonuses from campaign bosses is good, but forgettable. You'll only notice it if you forget the spirit bonuses.
To make the bonuses less forgettable, I'm suggesting we have a trophies board made from the now divine and enchanted tree The Stranger was impaled onto, to make us think about them by showing them to us through board management. The tree was effected by the way The Stranger was sealed onto the tree. Each trophy we get from a boss can be nailed onto the magical blessed board to give us bonuses through victories. Heck, even the tree itself could be the trophy board because of its unique event.
Minor trophies are swappable, can have a limited number of them active at once, and provides larger bonuses than the current ones, and helps fix attributes and resistances, and more. The maximum number of active minor trophies and minor trophy effectiveness upgrades through the campaign and end game.
Major trophies are all always active, more static, and provides core progressive character power by ways of adding sockets and levels to skills from items and ascendancies, spirit for spirit gems, and additional skill gem sockets for equipping more skills, and more space and effect for minor trophies. Do consider though, that I'm not asking for more, but asking to add a progression to get to what we currently have in the end game, for example to the 9 equippable skill gems and to the 100 spirit.
Trophies are static untradeable quest items, and are more indicative of exiles progression through the challenges. Similarities can be drawn to the Pantheon system from PoE1. The player will get the trophies either during the campaign by killing bosses, or from the same bosses in maps. Some trophies could be only from maps or towers, to leave some character progression to the end game. For example the permanent 5th socket to all skills from items and ascendancies, or permanent level 20 to all skills from items and ascendancies, with major trophies.
To entice players with collecting trophies, an empty trophy slot is most effective to trigger the primal urge to get completion done. Add just enough information of what is missing, and the players will do the rest.
Upgrading trophies is... maybe unnecessary, but not an impossible thought. One would either need challenges or currencies to upgrade them, like Exalt slamming, but if we get the promised 100 unique bosses, each could probably play their own unique part within the board and trophies with possible upgrades to any of them. For example I would prefer for us to add bones, bark, candles, incenses and other art to the board itself to increase the effectiveness and space of minor trophies.
The UI for the trophie board could be accessible from the map view for example. A full screen view of the board and swappables sounds effective.
That's the idea. I hope this sparks interesting discussions, and am waiting for your thoughts.