NURBS is a generalization of B-splines. I have also heard of T-Splines which potentially sounded like generalizations of NURBS, or more general than them at least. Wondering if there are anything more general than NURBS. If there's nothing more general, then knowing other things that are equivalently as expressive would be good.
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T-spline surfaces are indeed a generalization of NURBS surfaces. A NURBS surface is a rectangular array of Bezier patches. A T-spline surface is again a mosaic of Bezier patches, but these patches do not have to be arranged in a rectangular array; they can meet at "T" junctions, too.
As far as I know, there is no such thing as a T-spline curve, so it doesn't make sense to ask whether these are more general than NURBS curves.
Among the curve types in common use in graphics subsystems, drawing programs, and CAD systems, NURBS are the most general. Any curve that you're likely to encounter in these domains can be represented in NURBS form.
The class of implicit curves is larger than the class of planar NURBS curves, in a sense. These are curves represented by equations of the form $F(x,y)=0$. Any rational Bézier curve can be converted to implicit form, but the opposite is not true. So, you could build a spline curve by stringing together implicit curve pieces that would not be representable in NURBS form. See this question or this one for more on this topic.
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