How about these guys for the Aztec, Dutch, and Portuguese: Cuahtemoc (led a resistance after the death of Montezuma), William III of Orange (he married and co-reigned with Mary II in England and had a successful war against the English), Henry the Navigator (important Prince who helped spur on Portuguese Navigation, though he was never a monarch if I remember correctly) or if you want something more modern go with António de Oliveira Salazar (the dictator who established the Estado Novo).
Henry the Navigator is definitely a good candidate for Portugal, there is a very well made leaderhead for him too. Basically the main reason I haven't added more leaders to these two civs is that I know very little about them. I've made a note of your suggestions and when it's time for 0.5 I'll look them up.
The Aztecs are a different situation. I'm considering expanding them so that they represent more of the people and history of their region, rather than just the final pre-colonial empire. They'd start with Teotihuacan rather than Tenochtitlan for example and I'd probably rename them the Nahua (the name of the wider ethnic group including the Aztec, Toltec, Tlaxcaltec and more) or possibly even Mexica/Mexicans. I'm holding off choosing additional leaders until I decide for sure what I'm doing here, though Topiltzin will most like be one of them.
As far as other Civs go: Tupi, Zapotec (though I think this may be too many Mexican Civs), Olmec, Mapuche, I'm not sure if you have a Scandinavian Civ... or something to represent the Kalmar Union, and of course there are various North American Indian tribes you could add. It'd be hard to choose just 2 of these.
I did start work on an Anasazi/Apache civ back in 0.1 but I gave up on it as I just couldn't get enough decent art together. That's probably the only North American civ that's a strong enough candidate for inclusion. I'll have to see how the Aztec remake goes before considering another Mesoamerican civ.
I'm leaving Scandinavia to be represented by the Vikings (I should probably rename them). They're likely to get a third leader at some point, someone post-Medieval like Margrete I or Gustav Adolf.