CO leaving is a huge blow to the NYC operations of SkyTeam. DL has some operations at JFK but nothing close to what CO has at EWR. Plus DL is shutting down a number of their JFK international routes later this year.

VN is in negotiations to join which will also help a bit in SEAsia and hopefully for ST KingFisher will join. I would not expect DL to start operations to Oz anytime soon, despite their announced desires to do so. Getting Air Tahiti Nui on board would help in Oceania, but still leave ST as a distant third behind OW (QF) and *A (NZ) in that region. MH won't happen anytime soon either, IMO.

For South/Central America, Copa leaving is a big gap that would need to be filled. The other options listed are viable, but SkyTeam is very slow in getting partners online in the program so there will likely be some period of time where there is a void there.