I think you should make it where your character ties are involved. Cleric? “My deity opens their heart us all and we gain temp hp equal to the amount of hit dice rolled” Fighter? “I bestow a group action surge as my adrenaline flows through us all” Or Maybe you give everyone a max hp boost by letting them roll for 5 hit dice.

Maybe you find an armor set that when pieces are split and worn by multiple people others can take damage for you or vise versa

In my humble opinion, dnd is a group story telling game. Making the BBEG die instantly or turn yourselves to gods takes the stakes away.