Hm, as for MP controls, a gamepad/joystick would be plenty for each player, and in implementing that, you'd have controls set for console ports. Point the joystick the direction you want to shoot, and press a button to fire.
I can see plenty of interesting modes for multiplayer unique to the Osmos theme.
First idea: Take a random scattering of motes, assign each a size and vector. take a copy of that, rotate it 180 degrees and paste is down again with a different texture (for more people, rotate and place the configuration 360/p degrees, p times) . Each player can only absorb their own flavour, opposites simply bounce apart. The winner is whoever lives longer, collects more than half of their motes first, creates an opponent mote larger than half (making the previous goal impossible), or the bigger at the end of a time limit. The result is a perfectly fair map with plenty of strategies. It's frictionless pool where your opponent is the ball, and bigger motes that would eat him are the holes. Or it's a race in an obstacle course, depending on how aggressive you feel. :
Another, inspired by hours of 'meditation' on F1-1 to eat the sun, forgive the astronomical terms. Each player is a satellite moon, around a binary system of huge attractor motes. one is clearly marked as the 'property' of each player, and the match starts with each player in medium low orbit in a large dust cloud. The goal isn't to eat the other player though, it's to make the stars collide, and have yours the bigger at the time. Gather, spit at your star. If you're in the lead, you'll want to do it when you're ahead of your star, slowing the orbit even more in the process. If you're behind, do it where you can give more speed to your star, and hopefully buy enough time to catch up before the gravity pulls them together anyway. Mote rules could be the same though, feel free to take a dangerous (and expensive, masswise!) close or far orbit to catch up or be caught up to, and bring the fight to them. Even if you destroy them utterly though, you still may not win, if they did their job well. This would even work with two teams of two! An optional setting to speed up apon one player/teams death would give a human element to the game as well. If you're way behind, it'll make you think twice about eliminating the compitition. If you're way ahead, give yourself to the sun, and watch your opponent frantically try to survive a universe slowly speeding up on them. Yeah, slowly, at the instant of death, even I'd want to give the opponent time to adjust, and have the accelleration slow down over time. for example, if he gets 180 seconds before becoming inert and allowing the final speedy collapse of the system, 15 seconds to 2x, another 30 to 3x, 45 to 4x, and 60 to hit 5x.
And that's just ignoring typical multiplayer modes! You've got random deathmatch, king of the hill, (Make random map, designate a small area near the middle. the more mass you have inside the area, the faster your 'win bar' fills. (More accurately, [your enclosed mass]-[every other player's enclosed mass]=[speed of win]. or make the area small, and if disputed at all, no one gets points.).
Another misc. idea: Put (weak?) repulsers, attractors, or whatever in, and if you absorb it, you briefly become one, until you metabolize it all and it fades away naturally. This would require motes being able to support more than one variable in their mass. The default motemass, the metaboliser, determines how fast special motemass is negated. the active special motemass, the more the stronger the effect, and the inert special motemass, which does nothing. You don't want to make it into default mass, or you have an exponential digestion, when a linear drop in power is more logical. inert to normal is fine, but at a slower rate.