Ottomans should get Tatars for sure. From a historical perspective the Crimean Khanate was protectorate of Ottomans for more than 400 years. The Crimean Tatar cavalry was always an integral part of the Ottoman army. Related to game play I don’t think it will be problematic too, it will only add a second cav archer variant. The card could be named as the “House of Giray” and could use Tengri Shrine models.
Ottos having access to Phanar, Sufis and Berbers as other choices are also accurate but could be problematic gameplay wise, making Ottos more OP. However developers could make those alliances cost a good amount of resources (like idk 300 wood 300 food age 3 card) to compensate Ottos possible access to those alliances (or adds only one unit from the mentioned civs depending on balance).
Sweden could get Vasa (without Winged Hussars for balance), in my opinion and this will also be good for game play because it will give Swedes a proper skirmisher option without requiring necessary going the merch path (Jaegers).
Cossack host should be added as a minor civ too definitely. Adding Cossack host as a minor civ makes so much sense because different Cossack hosts made alliances every major power in their region and fits perfectly to minor civ mechanic of aoe3. They can be available to, Russians, Sweden, Ottomans (also Poland in the future 
). It can have a similar mechanic like the Berben Sultan, giving access to a Cossack Hetman or Ataman who can create Cossack daredevils.
I saw a previous post on the same issue and I mostly agree about the native civs part that who should get which minor alliance so I don’t want to repeat it but don’t totally agree with the mercs part.
I hope devs will make this the norm for all civs so it will add more variety to gameplay for all civs.