It is possible that the developers already decided on what to do in the first Expansion.
Game development goes through different stages that need different parts of the team work on, so it’s very likely that parts of the team already work on an Expansion while others polish up AoE4 for release.
I linked the thread about Indian civilisations.
Most of South East Asia is culturally close to India because they are mostly Buddhist or Hindu at the time.
The umbrella of the Iberian Peninsula is just “Spanish”, the name with wich people and kings from the different kingdoms of the peninsula (with the exception of the kingdom of Portugal) called themselves.
AoE2DE (and maybe AoE3DE) seems to get 2 civilisation DLCs so lets assume AoE4 will do the same.
The number 2 means that it’s easier to make a DLC compared to 3+ civilisations and also it’s easier to stay on topic.
The African Kingdoms DLC had only 2/4 African civilisations for example.
Two civilisations can also be nice to show a rivalry like adding Turkey (Rum/Ottomans) and the Byzantine Empire as rivals.
It also allows to give isolated civilisations someone to interact with like Aztecs and Maya/Zapotec.