even better yet, ther ecould be an auto-download function that takes a sec to download the 'host's' map, and use it for the match. You could simply pull open Sue's workshop, make a map, save it seperatly as a map file, and open it up with the CaveStory Online... Then you could make maps ranging from a simple, 15x20 box, to a vast 100x100 laberynth.
Also, we could possibly have "spawn point" for capsules that hold a random item... such as a heart (heals 10hp), a heart-group (Heals 100hp), some exp shards, missiles, maybe even a few new tricks like a 'blinder' (all opponents are surrouned by darkness, leaving a 5x5 visibility circle), a 'teleporter' (all opponents swap locations at random, or maybe even a barrier, that could absorb all attacks for a limited amount of time (say... 15secs).
As for modifying weapons, you don't really need too. The polar star would be available to people who want a challenging fight, the fireball would be fun, firing off a cliff randomly, while your opponents have no idea where you are, the bubbler is a nice defence for defending against the sword, level 2/3 nemisis, polar star, fireball, possibly even missiles. The snake would pwn for large maze-like stages, while the super-missiles at level 3 would be deadly.
As for presses, they'd be a nuesence, dishing out an uneven number like 127. Maybe lowering the damage to a nice even 100? Even so, I don't think I would enjoy playing a stage with presses in it. You could be down to 50hp, a hit or so from your opponent would end the match, while at the same time you could win if you're stealthy... then BAM!! A press nails you.
Another Idea would be a possible weapon-switch in with the capsules. Maybe players COULD all use the same weapon... a mass missile barrage coming from all directions would be... interesting.