Adding all of the mechanics on top of each other would cause power creep and bloat. Gameplay loops from the seasons should be evaluated to add to the base game just in a nerfed state that keeps power in line with the current season.
If the game had trading they could have made these trap dungeons drop amulets or w/e that were in line with the current power ceiling. Then these dungeons could be kept as part of the base game. Those who enjoy the trap dungeons could then run them with specialized builds next season and trade away amulets to those that did not enjoy them. It is a simple way to create an economy and make the game better over time. The braindead anti trade approach really kills the potential depth of the game.
I think they should add something each season that can be taken back to Eternal where that power creep doesn’t matter so much. It would be an incentive to max out each season so you could build an Uber character on eternal eventually.