Eliminate bosses from maps and put them on every tower. More to this idea.

In video you said you originally intended to make bosses on every 4th map on average. Make the towers on average every 1/4th map and at the end you have to kill a boss to unlock precursor tablet. Also you could tie these boss kills on towers to add to your waystone level drop like you did in poe 1 with conquerors and watchstones.

Bosses are well designed and make the game more fun so players should be interacting with them more than just for loot. Having killing them as the tower boss and increasing your passive waystone tier level would be a way to make interacting with them more rewarding.