1) other than the big 2, your best bet is the Frankfurt Parliament, a self-unification, as it were.
2) I like the idea of a Hannover based one. British backed to increase trade up and down the Rhine (to start with), building on alliances and marriage ties. (So, Brunswick, some of the Hesses, etc., to start with.) By 1820, the Hannoverians were still quite German. Younger sons were still acting as viceroys until Victoria. Have an earlier Salic law split (basically a slightly earlier Victoria) and a Cadet line gets Hannover, and leads the unification. First as a non-Prussian Customs union, later as a wider federation.
3) with an earlier PoD, Saxony could do it.
4) wilder ideas include Gustavus Adolphus (Sweden) or Christian (Denmark) uniting Protestant Germany (at least) during the 30 Years War, which then becomes all of Germany later. Note that these would be less Sweden or Denmark unifying Germany, and more their kings leading a native German coalition. Could still end with a Scandinavian monarch, but Sweden, at least, won't be part of the Reich.
5) wacky idea - genius Danish king manages the Schleswig-Holstein question so deftly that DENMARK unifies Germany.
6) totally wacky ideas. The Netherlands. The Hansa.