There's discussions in some places considering whether it might be worth it to try and build alt-chans to service former 4chan communities, but I don't know of anyone who's trying to outright replace the whole site.
It will never work because 4chan fills a specific niche;
the need to scream into the void
semi-anonymous communities are good to have, and there's a place for forums like this, but what made 4chan so successful AND so influential is that it is a bunch of howling retards and once in a while everyone needs to be a howling retard. It is not tapped into the collective consciousness, nor directs it-- it is that collective consciousness itself, free of filter or fear. The core reason that 4chan is always right is that it is where you go to make a beast of yourself to let go the pain of being a man, and find new insights from the crumbs of sanity inbetween rampant unrepentant nearly schizoid babble.
A key reason 4chan could do that is by the time that insanity was proliferating, it already had the userbase and thus posting velocity to enable nearly real-time conversation, and almost none of the usual potential punishment for wrongthink or simply being really fucking dumb. All the things you could say on a normal forum but have fear of targeted reprisal or say somewhere else mostly anonymous but not get a usefully immediate response could be said on 4chan and get that immediate engagement. And not engagement like "I will get updoots" or "I will fight you and win" but straight up chunni-ass "we will fight eachother and grow from this experience." An extension of "this is what they actually believe" to "Is this what they actually believe? Is this what you actually believe? Is what I actually believe what I actually believe?"
There have been papers written by people smarter than me on the tactics and motivations for trolling, but they mostly boil down to either saying something provocative to intentionally confirm a pre-existing bias, or asking a known-dumb question in an especially dumb way to garner a useful response where simply asking the dumb question in a smart way would be ignored.
Altchans can't fill that niche because they simply aren't that big enough to have that immediacy. 4chan is the equivalent of hobos ranting on the bus. The hobo always exists, but the bus gives him the place to rant. Sometimes we want to be that hobo. Sometimes we need that bus. But unlike a real bus, an online community that has that critical mass of listeners who will engage you back can't be made, it simply comes to be.
And yes, I am aware that I am being a howling retard.