He wants nothing but a little more liveliness, and that, if he marry prudently, his wife may teach him.

Translated into modern North American English, this would read as:

"He only needs a little more liveliness." Another way of putting it would be "lack" - i.e: at the moment Darcy lacks liveliness, but that could be improved if he married the right woman(!)

JA also used "want" in the modern sense of "to wish for something", but here are some other examples from the book of "want" being used in the stronger sense of "need" or "lack":

The young man wanted [needed] only regimentals to make him completely charming. (ch.15)

Mr. Darcy can please where he chuses. He does not want [lack] abilities. (ch.16)

[Elizabeth] found herself suddenly addressed by Mr. Darcy, who took her so much by surprise in his application for her hand, that, without knowing what she did, she accepted him. He walked away again immediately, and she was left to fret over her own want [lack] of presence of mind. (ch.18)

There can at least be no want [lack] of subject. (ch.18)