I hate the idea of Bijuus being simple bosses, instead, being a boss and being able to capture it, thats another story (once captured it disappears). Adding sealing/Stunning jutsus (depending on the player's power/mastery of it) to stun The bijuu and other squad sealing it into someone while its stunned. Now, for Dropping stuff, I've never been a fan of monsters dropping equips, its not logical, but yes about dropping materials for crafting equips, if its kyuubi, as example, it could drop a piece of meat that you eat and gain some kind of chakra boost, hair that you use to craft whatever kind of equipment, a fang/claw it drops to craft any kind of weapon, thats another story. Bijuus having to be captured to get someone to be a jinchuuriki seems way funnier to me, it implies much more responsability, as you're gonna have to be a really strong player to be chosen as the jinchuuriki, because you'd be in constant risk of getting killed/captured by anyone (When they kill you, as example, the body remains there for a certain amount of time, IF unsealing/sealing transfer jutsu is used, you lose your bijuu).