Expanding to 20 cities will require annexing the other Mayan states and Zapotec, so forming an alliance with future prey also means waiting through a truce timer after dissolving the alliance. Right at the start, the player should focus on making allies by improving relations and royal marriages. then all its provinces are removed from the HRE. Huastec is a Mayan country in North America, separated from the rest of their fellow Mayans by Nahuatl states. Increasing autonomy of distant heretic provinces will be a good choice, because after they become independent and are conquered again in next cycle, the autonomy will drop to 50% (40% with a claim). Mayan provinces start out poor and being a primitive nation leaves them with no institutions, reduced gold income, increased development costs, and bars them from building navies; in addition, the Mayans can only colonize provinces that are directly adjacent to fully-settled core provinces. Therefore, everytime you pass a reform, you'll be severely weakened. The -20% coring cost modifier is also optimal, but keep in mind you're going to stack most of your admin points without spending them on technology.
This should be your first target. Further expansion in the Americas will rely on extensive colonization, which can be done in tandem with conquests of other regions. When they arrive, the player has to act fast, improve relations with them, and help them in their inevitable conquest of the Inca. God damn beautiful.
Researching any further is a waste of points until reforming the religion. The Mayan cycle of expansion and contraction will cost many, many Administrative points to get through due to coring costs and the occasional need to raise stability after passing a reform. It can be formed by states with their primary culture in Mayan group and the Mayan religion, namely Xiu, Itza, Cocomes and Kiche in 1444. Later expansion into East Asia can also be a good strategy since the technologically-lagging Asian countries will not be able to offer significant resistance, with the possible exception of Ming if China has not disintegrated into civil war. Keep in mind however, that Nahuatl nations are less likely to accept your alliance offer so a useful Mayan ally might come in need. The lost provinces will usually be given to nations with cores on them, particularly annexed countries which will be liberated when the reform is passed.
The League is free to choose any direction to expand in, though it is a good idea to hold off on colonizing/invading Asia until more of the New World is secure. Upon passing a religious reform, your nation will lose 1 stability, release all of its vassals, and be reduced to a number of provinces equal to 10 + 2 per religious reform already passed (so 10 for the first one, 12 for the next, and so on). Exact provinces released are determined by culture, religion and distance to capital. If the economy is sluggish, ‘A Unified Army’ can help raise income; if the economy is fine, ‘Central Arbitration’ mitigates the constant unrest in conquered provinces.
Like all New World countries, the Mayans begin at level 1 in all technologies and, lacking institutions, the Mayans begin the game with a +50% research penalty. This makes rapid occupation and assimilation of European colonies a wise strategy, so engaging in as many wars as quickly as possible can nip the Western empires in the bud and secure large amounts of new territory for the empire; almost as important is the blow to European power, the greatest threat to the continuation of the League.