Many times when I attempt to build a mob farm I often spend too much time creating a perimeter so that the farm actually functions. This process is time consuming and very boring. My proposition to fix this is monster platforms. The first step in creating these would be to gather spider eyes, bones, rotten flesh, gunpowder, slime balls, and ender pearls (the drops from the most common overworld monsters) to craft into monster essence (you would get 2 from the recipe). The next step would be to surround endstone in 8 Monster essence to create 4 monster platforms. Monster platforms would function by having an increased priority when spawning mobs, so mobs would have a higher chance to spawn on the platforms than anywhere else. This would make perimeters less important. This block would be able to be moved by pistons and I imagine that it would be an light yellow color with a creeper face on the top.