There are a dozen games like Metaball, they are all derived from each other. Switchball for example, Marble mania kind of as well, and so on. The element they all share is the ball yeah, but to me it's closer to Portal than to all those ball puzzle games.
I usually don't really like those typical ball puzzle games because you have so little control over the ball, and usually you just roll yourself into a hole somewhere.. My approach is to give the player a standard fps character, of which he has maximum control, and have the character mess with the ball. If the ball ends up in a hole or some lava, big deal, just pull it back in. HL2 Gravity gun meets tomb raider meets portal meets metaball.
Most ball games are usually also quite bright, cartoony, and childish. Mine will be dark, slightly less cartoony, and a bit more mature. I dont have the time and manpower to fully develop the theme and background story right now, but the original idea was to have some bloody action bits, which would also be quite satisfactionary btw, killing monsters with a big ball, and some scary elements like a mummified creature that "lives" inside the ball you are playing with.
We are starting simple though..