To get significantly better/faster than the DA 35 2.4, you'll have to spend a lot of money. And even then, it's probably going to be a personal taste thing. I don't know if Sigma 30 1.4 will have better IQ, just different and quite a bit faster, of course. FA 31 would clearly be a step up. Same with the Sigma 35 1.4 ART (some people don't think it's that great, especially bokeh wise, but I think it's fantastic).
Me, I'm perfectly happy with the DA 35 2.4, same as the one you have, for chasing the kids around. I've got some really nice pictures taken with it, so I'm good for now. If I get rich, I might think about spending 3-4x as much for another (arguably) 5-10% improvement in quality
Oh, and there's the FA 35 f2.0 as well. KEH has 3 of them at the moment, in the 325-349 dollar range. Might be worth trying out. Some people like it better than the 35 2.4, and it is definitely a bit faster.