Does any team from outside Argentina or Brazil have a chance of winning it ?
At the moment I would say it is highly unlikely. Flamengo look like they are trying to build a dynasty so it is really hard to see past them.
I had a look and only 4 non-Argie/Brazilian teams have won the Libertadores since 1992.
List of Copa Libertadores finals - Wikipedia
en.wikipedia.org
Flamengo 4/1
River Plate 7/1
Palmerias 9/1
Boca 12/1
Gremio 14/1
Only 1 team in the top 8 in the betting not Argentinian/Brazilian - LDU Quito at 18/1. As you said Flamengo favs mate.