On the floating rocks looks good but too popular. Maybe you can make a pro base into the ruined castle under the vulcan, but music will kill you:) Or into the underground ab cave, or into the grey rocks where the organic polymers are growing, that's a really good point:) You can try the corrupted island too or the back side of the Ice Wyvern's island...
Or you can find a secret tunnel in the Wyvern's trench.
I don't know :)

These are our bases of amissa, pve:) Simple locations just for inspiration:

1. The Big Castle's location is my favorite maybe a little opened, 2. The Catedral is on a Floating rock, and a 3th mini on the side of a cliff, and a basic on the beach. Too easy but really massives with enough places for pvp..

Sorry for the spam:)

> The Castle
https://i.ibb.co/2hh9XCw/20200119110516-1.jpg
https://i.ibb.co/jfkY1wg/20200203211717-1.jpg
https://i.ibb.co/2hXZ3Nh/20191227222311-1.jpg
https://i.ibb.co/p2xNSqX/20191215172218-1.jpg
https://i.ibb.co/mJvXtz3/20191227191529-1.jpg

> Floating Rock Catedral
https://i.ibb.co/yRCCjFY/20200116204519-1.jpg
https://i.ibb.co/Ptx5hRX/20200109202741-1.jpg
https://i.ibb.co/GkMvcPZ/20200116205925-1.jpg
https://i.ibb.co/3sXkDFK/20200105161847-1.jpg

> Really safe :D
https://i.ibb.co/MPsHR95/20191210223203-1.jpg
https://i.ibb.co/pQNfc2s/20191210222914-1.jpg
https://i.ibb.co/pKx79zm/20191211202501-1.jpg

> The Beach
https://i.ibb.co/17Z6MQV/20191026155037-1.jpg
https://i.ibb.co/Dk1VFsJ/20191027115707-1.jpg
https://i.ibb.co/wdkhJL9/20191026164827-1.jpg

On the beach you can find a perfect rockcave. You can close this with walls and 2 giant gates perfectly. Not too secret.. rather safe.