i suggest... i suggest taking a walk through campaign and looking at the structures within there. incorporate some forerunner structures into your map, and maybe others. i recommend playing seirra 117 for basic ideas if you are new to interlocking or just forging in general.
use boxes to make a random layout, then incorporate bridges, walls, dumpsters, second levels and respawn points and BAM u got an awesome slayer match. but its only awesome if u think about what u are doing while you do it, and use merging to make it cleaner and look prettier. do this every time u have forgers block and it should help spring ideas back inside ur head.