* American Airlines: this was a very loyal carrier for many decades. This is a must have carrier. New York and Chicago should be added again and they even can use the A321XLR. Extra destinations would be good with them.
American Airlines is currently expanding from its Philadelphia hub. I see that there are 3 airlines, from BRU to NYC, on this route, it's probably enough. PHL is also less saturated than New York, the competition is less stiff (no UA or Delta Hubs), and you can reach, many cities in North America and South Africa via PHL.
LATAM: South America is a must have. They are flying already with cargo to BRU. Pax would be the logical next step. Brazil and surroundings are high on the list.
I don't know how is market between Brussels and South America, but if a carrier starts a cargo service, that dosen't mean, that they will start passengers service. Both Qatar Cargo and Emirates Cargo serve some destinations only with a cargo service. Also, an airline can serve a big market in term of pax, but can't provide a cargo service to these destinations
It's true that AF and KL will more and more use train connections, but BRU should try to attract travellers by train as well. This is done for the Netherlands, but not for France.
Not only AF KLM. TGVAir has partnership with Air India, Qatar Airways, French Bee, Air Transat, Air Thaiti, Air Caraibes , Air Austral, Etihad, Emirates, Vietnam Airlines Royal Jordanian.
See the list, at the bottom of the page: https://agence-voyage.oui.sncf/vol/tgvair