So the steel path interests me, but what interests me MORE is the fact that they have said this is a step towards end game and not to be conflated with endgame itself, which reads to me as "we know the steel path is artificial difficulty for now, but enjoy this while we make actual end game content."
So to me, combining the steel path with the addition of rathuum and index style enemies would be a huge step in the right direction.
Things on the floor to avoid, attacks you HAVE to cover from, enemies you HAVE to prioritize, slow moving projectiles you HAVE to shoot up or dodge. That all smells like end game to me.
The Jupiter tile traps were also amazing for me personally, I loved that the environment had ACTUAL lethal hazards rather than "oops the door turned on and I got knocked down and have no energy"
But I feel like the biggest obstacle before "end game" or at the very least ENGAGING content that DE has to overcome is enemy design.
Traditionally in horde shooters, especially endless ones, the majority of enemies would be low mechanic requiring (IE shoot them and they die) that are not very dangerous until there is a lot of them, but woven in the waves are heavy enemies with a lot of lethality and skill to take out. For example in killing floor there is a MASSIVE difference between a clot, a siren, and a scrake. and it seems like way back when DE WANTED to do something like this, but executed it poorly, as there is very little difference between a lancer, a bombard, or an ancient healer. They may be tankier and attack ever so slightly differently, but you don't have to deal with them any differently than you would other enemies. you just shoot them and they die, typically before they can even be a threat.