Recurrent complex is great for that, it's like ruins but with cooler structures and better integrated generation, depending on your modslist I even have some custom structures you could use
Looks promising, but is there a way to get a complete list of the default structures without digging through the jar? Visual references would also be nice. Knowing how to whitelist or blacklist them in the configs is useless without a master list to work from. As a custom pack novice, this is a crucial first step before learning to make my own.