The most viable unmade character--even more so than Cordy, IMHO--is Tara. People underestimate the popularity of the Willow-Tara relationship, which was actually the only healthy romantic relationship on the show {not to mention an historic milestone in gay/lesbian portrayal on TV). Additionally, her OMWF dress is iconic not to mention gorgeous, and a Tara in that outfit would be a huge draw for multiple audiences much as the lovely attired Drusilla was. If announced in conjunction with a long-overdue decent Willow figure (Dark Witch Willow, to get the interest of multiple audiences), it would reawaken and rejuvenate interest in the line as well as correct Sideshow's worst mistake (that ugly turn-off line-hurting travesty of a Willow we have now).
As much as we all love Cordy, she just doesn't have the gorgeousness and drawing power that an OMWF Tara would have esp. if announced in conjunction with a decent Dark Witch or other Willow. DST's OMWF Tara and OMWF Willow interlocking busts have been selling briskly and tapping into a beloved dynamic Sideshow has failed to exploit
That renewed level of interest would in turn make Dawn, Anya, etc. viable in future. Lines don't just sustain themselves in a vacuum; interest must be built through a good variety of release choices designed to appeal to different segments of the fanbase. Sideshow has been very uneven in that respect ever since alienating fans and retail partners with the ugly Willow.