Hello MinecraftForums! I'm here to share CivClassic, which is a great civilization building server. It is semi-vanilla with most main features the same, save a couple key plugins I'll explain below. I included PVP in the tag but the server doesn't have a PvP emphasis, it just doesn't restrict it so not quite SMP. Most nations have laws protecting against attacks and police action to get justice.
The central idea of the server is player-led nation building. Groups of friends get together and form their own nation, often picking a name, flag, set of laws and government. These nations interact with each other in the form of trade, war, building, etc. The most populous nation is a dense city state with unique democratic politics, or you can find many other nations which have lots of room to build and less emphasis on politics. You can explore and find what you like, and play how you want to!
Some core plugins:
Citadel - Allows users to reinforce blocks to be harder to break
PrisonPearl - Allows you to inflict justice--or vengeance, by killing a player while holding an ender pearl on your hotbar. Use mana (earned through daily logins) to fuel the pearl, either exiling the player (they cannot start a fight, or grief, etc) or imprison them in The End (they cannot escape and are trapped until you free them)
Bastion - Allows players to protect against grief by placing bastion fields that restrict block placement
JukeAlert - uses Jukeboxes and Noteblocks to alert you of anybody breaking your blocks or using your doors
FactoryMod - allows large scale production of many materials and high investment, but greater rewards for vanilla actions like smelting ores
This server definitely benefits from teamwork and playing with others, so I always recommend joining a nation or coming with a group of friends to help learn the ropes. The admins are hands-off, and no hacking or cheating is allowed. There are resources on the subreddit information about the server, and most nations have their own Discord servers. There's also a Mumble server.
We have a ton of really committed people that make things just for the server, checkout the recently launched CivBook website which archives books from all of the different iterations of the server! https://ccmap.github.io