The Darkage mod can be added into an existing map but ores and stones will only appear in newly generated areas I think.
It has lots of variety and its organized so the user can disable stuff they dont want to use.
The ore generation is kinda dated compared to the current method,
but I still use it and have had no significant slow downs or errors using with the latest Minetest game.
http://nimg.pf-control.de/MTstuff/modSe ... &q=darkageMasterGollum's is the original.
Craigy David's has been updated and accommadates more recent/popular mods like moreblocks etc.
There is another more recent mod called More Rocks by Thomas Brod
its still a W.I.P. but has nice results...but it has to be used from the start I believe.
viewtopic.php?t=11779