As boss arena was suggested ealier, i thought, why not an enemy battle arena?? But we should have several different modes to it and several stages. Here are some of my suggestions.
Survival mode (Choose your stage, and you will be swarmed by enemies until you die) (BTW, this would be great if you could choose how many enemies at a time, and which ones).
Time attack mode (simple, you have a timelimit, and see who can kill the most eneimes, scores should be posted on the site. Also, they should have multiple difficulties with harder enemies).
Door 2 door battle (go into a room, fight an enemy, go to next room, fight harder enemy, eventually repeats with more weaker enemies, and health every few rooms, this should go on for about 99 rooms to make it hard).
Race to the finish (similar to SSBM race to the finish, just with enemies). Only quicker enemeis such as pirates should be in this, and will chase you).
Samus brothers stamina(you choose from a few stages, and fight 3 other enemies, pirates, chozo ghosts 3etc, that have much more stamina, the same amount as you, more on hard mode. Records will be kept. You can choose the CPUs.
Samus brothers melee (same concept, except you must knock your opponents off the edge, though i relaize this might require some extreme physics engine modifications).
These should all be unlockable by doing varios things in the game (mabye finding special trophies hiddin in secret rooms, or beating secret bosses). While most of this has already been suggested, please consider them. Im a smashgeek, so if this is straying too much from the formula dont mind me.
BTW, if this gets locked…I will cry (feel free to quote me).
if wanted, i can make that, it might not be part of the metroid game, but it will be fun anyways.
though i have to finish the samus engine first (it is getting a lot better now that you can aim, shooting is a piece of cake, all i really need is to add health, and then add different types of beams, then i done!