I agree with Joost's comment about the new route - it is more a legacy and a great experience waiting to happen for the motivated, than it is about access to new habitats/birds.
Ignoring birds that are hard to get on your Chile list, which is a very different discussion, globally hard birds and major Chilean targets that are genuinely hard would be:
Fuegian Snipe: this is one of the harder birds in S America. If a good and accessible stakeout were found it would be a major feat. I'd put my trekking shoes on if I knew I could see this species!
Rufous-tailed Hawk: kind of hard but there is usually a nest known and with a bit of time you will see one.
Blackish Cinclodes: readily gettable via a boat out of Ushuaia so doesn't quite make the cut for me.
Chilean Woodstar, Ticking Doradito, Horned Coot, Diademed Sandpiper-Plover, Ruddy-headed Goose, Magellanic Plover, and many others are certainly great birds but are not really that hard and sites are well known.
Hooded Grebe is essentially a vagrant to Chile as far as I understand, and is readily accessible in Argentina with proper preparation (and is one of the best birds in S America in my opinion, I was not prepared for how gobsmacked I was when I saw it).
Masafuera Rayadito is one of the hardest Chilean species, and certainly is one of the more logistically challenging species of note in S America but isn't really germane to the discussion as I'm pretty sure the trail isn't going to give way to a surprise population of this species