Walk off edges are illegal (bridge of eldin). Anything with walls allowed for infinite back in melee, and though I've read that there are no longer any infinites, I doubt the community will learn to accept any stage with a wall. The majority of stages employ a triangular platform layout (like battlefield), which is a tried-and-true standard for smash. The problem with making a custom stage with a triangular platform layout is that it might not offer anything new. If you make the platforms too high, camping can become an issue (kongo jungle 64). Dreamland 64 is known as having extremely large boundaries, favouring characters with effective recoveries, so unless you have some other convincing element in the stage, I suggest making boundaries that resemble battlefield's. Lastly, make sure that the stage doesn't have too much going on or else it may lag in in doubles (skyloft).