I think that Lithuania could have a great story that could be able to fit in right on assassin’s creed universe. In the 14th century, the Teutonic knights were in control of the majority of northern Europe, As Poland being a major Christian force(Templars) trying to invade Lithuania because it was one of the last non-Christian countries in Europe(may be because the assassin's kept it that way). In the 14th century Lithuania adopted Italian architecture while still keeping some of the early gothic buildings that were already there. There were a lot of political assassinations involving the Teutonic knights or knights Templar. Possible targets could be rulers such as Vytautas the great, svitrigaila, sigismund kestutaitis. Casimir IV jagiellon, Alexander I, sigismund I, sigismund II and sigismund III could all be possible assassination targets
The act of personal union with Poland was signed as early as 1385, however, continuous line of common rulers of the two countries started only with Casimir IV (even then Polish and Lithuanians twice selected different rulers following earlier common monarch's death, but the Lithuanian one always eventually assumed Polish throne). The monarchs retained separate titles for both parts of the state, and their numbering was kept separately. The jagiellon dynasty was a direct continuation of the gediminids.
"Witold wanted to be crowned king of Lithuania; but before this could be arranged, he died on October 27, 1430. He was succeeded by Jagiello’s brother Svitrigaila, who resented Poland reclaiming Podolia and made an alliance with the Teutonic Order. In 1431 Poles went into Volhynia and besieged Luck, but an armistice was declared. When the Teutonic Knights invaded northern Poland, Svitrigaila renewed his alliance with the Order. The Poles supported a coup d’etat by his enemies on September 1, 1432, but Svitrigaila escaped. Witold’s brother Zygimantas Kestutaitis became Grand Duke at Vilnius, and at Grodno on October 15 he made a treaty with Poland settling the Podolia frontier dispute. Poland defeated the Prussian branch of the Teutonic Order in 1433."
This also fits into the same time of of the Prussian empire, in Assassin's Creed: Brotherhood at one point, Desmond asked Rebecca if she had ever used the Animus to learn about her ancestors, at which she commented that the lives she had relived were "lame" compared to those of Desmond's ancestors. She continued, complaining on how boring it had been spending the whole day firing guns as a Prussian mercenary. This might not seem like much but tied into a story where Desmond or possible someone else's ancestor goes to Lithuania it could have a tie with Rebecca’s ancestor where we could see that her ancestors aren't "lame" but a big part or they're at least in the order.
Weapons, the weapons in this era would most likely be the same as the weapons in AC:II or AC:B but would be a little more less advanced seeing that it would be around the start of the 14th century whereas AC:II and AC:B were towards the end of the 14th century. Some suggestions of weapons would still be things like swords, pole arms, cross bows, long bows, short swords and throwing axes though they would be made out of other materials first seeing as northern Europeans have been known to make weapons out of animal horns.
Guards, the guards could be the Teutonic order or they could get help from other European countries like the Borgia got help from the Swedish papal forces.
like the majority of other European countries it is ritch with art and history so you would be able to buy paintings and books for the hideout like in AC:B, AC:II and AC:R.
this is just an idea of mine after i came home after my holiday to the Baltic states, seeing some of the architecture saying to myself "hey, i could climb that". This is just my opinion that i thought i would share.
Good architecture, war, political distress= Assassin’s Creed?