Any country other than Belgium would be good. It would be a bummer if both minors were countries that spend the entire war under occupation, and unlike the Netherlands, Belgium doesn't even have extensive overseas holding or much of a navy of any kind.
Mexico, done right, could be interesting. It has the power to be a kingmaker if there is a civil war in the US, not to mention that newly revealed mechanic of countries being able to own the resources of foreign states could play a role vis-a-vis the whole oil nationalization crisis in 1938. There is plenty of setup for fun alt-history given the power struggle between President Cardenas and former President Calles. Historically the latter was exiled and his supporters purged kind of early in 1936, but what if it had gone the other way?
If the second minor is going to be a country in the Americas, it won't be Brazil. I think Brazil is a good candidate for new content, but the issue is that it would be pretty sterile without, at the bare minimum, Argentina also getting new content. Mexico, in contrast, doesn't really need any additional content added to other countries in Latin America to be fun.
That's also the issue in Europe. I think Norway would probably be a better candidate than either Belgium or Mexico, but that also runs into the same issue as Brazil: unless content for Norway is developed alongside content for the other Scandinavians, it probably won't turn out that well, so it's probably safe to count out Norway.
Don't get me wrong, content for Belgium would be good, but in practice it would probably just play as a less interesting, less flexible Netherlands with the same issue of being occupied the entire war in any even semi-historical campaign. It would just offer a very similar gameplay experience. I think offering a diversity of options and varied gameplay experience is more important than doubling up on countries that would play mostly the same. This is a problem that DoD suffered from in my opinion, and potentially, any future 'country pack' with the Sweden-Denmark-Norway trinity. The only problem about not getting Belgium now is that we will probably never get it if we don't get it with this expansion.