A plug-in that just lets you increase the spawn rate of mobs per world, my spider fill forest world need more spiders. All the plugin I looked at were mostly for zombie and let you modify them. 1.You can set the rate of a mob in game 2. Set how many mobs will spawn per spawn point 3 Set mob spawn points, were you stand
Not sure about setting spawn points at your location but OtherDrops can replace natural spawns, eg. make spider spawns drop 2-5 spiders each rather than just one, or replace some zombie/skeleton/creeper spawns with spiders (either in addition to or replacing).