Basically this: MERCOSUR/MERCOSUL is, in OTL , a Free trade zone that should have become a Common Market (it is even in it's name). Unfortunately, this economic bloc falled into stagnation and the Pacific Alliance is going better in South America. So, how could MERCOSUR and it's have done better economically and politically to prosper? And what new countries could enter in this bloc? And the other consequences (for the member countries and the world)?
That the founding members don't be so tight with their trade and economic special interest, it´s supposed that the Mercosur it´s a economic alliance (Mercado Comun del Sur/Mercado comum do Sul) but it´s more dificult negotiate with the Block than with every country individually, there is a reason that Chile, as example, Have economic accords with all the countries part of the Mercosur individually, but not accord with the Mercosur, the Group, create to make easier the economic trade and integration of the region, in fact make all the process more difficult and put more tariff than any individual country, there is a Reason Peru, that have all his economy orientated with economic trade with the Asian-Pacific Region don´t want to be part of the Mercosur, even after being invited at the time.
There is a reason that even with the Mercosur being more rich and with a Bigger economy than the Pacific Alliance, the Pacific alliance is more popular and have a bigger overall economic export production and more direct investment between the countries of the Alliance , and their economic ties.