The player’s country:released vassal.
Riga in 1444 starts off in quite a precarious position as they are the smallest country in the Baltic area. In close proximity to such powerhouses as Poland, Lithuania and Muscovy, all with interests in the same lands as Riga, they must act carefully in the initial stages of the game. Denmark and/or Sweden may also pose a challenge if they spread into the Baltics early. A good way to ensure Rigas independence lies in securing alliances with some of the earlier mentioned larger nations. Do not expect them to join Rigan wars from the start however, as the new favor system gives smaller countries favor at a slower rate.
One of the most viable options in choosing where to expand in the beginning lies in the lands of the Livonian Order, as they are Rigas closest and weakest neighbour. The Livonian Order does usually ally with the Teutonic Order and as such almost inevitably goes to war with Poland/Lithuania at one time or another over the Teutonic lands. When the Teutons are exhausted from the war with Poland and the armies of the Livonian Order are either crushed or low on manpower Riga can take the opportunity to strike and hopefully gain some provinces from the Livonian Order. After that the power balance between Riga and the Livonian Order is probably shifted, thus further expansion against them will not be as problematic.
Next off Riga can either expand into the remaining Teuton lands (and maybe draw the ire of Poland/Lithuania in the process), look to the east (and ending up in conflict with Muscovy/Russia) or to the north/west (and go to war with Sweden/the Kalmar Union). If Novgorod still exists at this point it can be strategic to take some of their provinces with high development and trade power to bolster Rigas economy, but this will almost surely anger Muscovy. If Sweden has broken free from the Kalmar Union they become an easier target for expansion, but their provinces are generally not very high in development. The increased force limit is however always welcome.
The Rise of the Rigans
From hereon, when Riga has established itself somewhat as a power in the Northern Baltics and/or Western Russia/Scandinavia, Riga has the possibility of attacking either Poland/Lithuania (or if they have taken the decision, the Commonwealth) or Muscovy/ Russia. With help from Rigas allies, which presumably is one of the aforementioned countries it will be possible to chip away at the enemy over time, eventually eclipsing them in power. In the off-chance that Muscovy was the ally of Riga the achievement is probably gained at this stage of the game as the Commonwealth will be losing their Baltic lands to Riga.
If it was the other way around Riga has gained a lot of development and income from the conquered Russian provinces which can aid them in later, inevitable wars against the Commonwealth.
Rightful rulers of the Baltics
The Commonwealth can be quite the challenge to take on due to their high income, massive force limits and possible allies. There are nevertheless some factors that aid Riga in their wars against them. Riga can get high discipline bonuses as they get 5% discipline from their national ideas, some 10-15% from ideas, policies and religion and an additional 5% from the Commandant advisor. This discipline bonus should not be underestimated. Another advantage lies in that Riga belongs to the western technology group, and as such has better late game units and an easier time keeping in front of the Commonwealth in military technology. Through some strategically planned wars, where the first war can be used to force the Commonwealth to annul treaties with powerful allies, Riga can probably beat the Commonwealth and arise to the position as the rightful rulers of the Baltics.