Recently, William Shatner put out a documentary called "Captains" that he hosted, featuring Patrick Stewart and Kate Mulgrew as the later captains of the Enterprise. William Shatner has 216 acting credits of which one, Star Trek, became an iconic show and spawned several movies where he played the same character. His career started in 1951 and continues to this day, a span of 63 years, but he is best known for his four years on the last 79 of the 80 episodes of Star Trek.
Buffy could do the same thing and have the same exposure. It is understandable that she would want to be able to move on from Buffy, but there is less harm than you might think from embracing the series and moving on. It may well be the case that her outstanding contribution to popular culture would always remain Buffy. That's not a bad thing. SMG could readily embrace her old character and still move on. The big career winners on Buffy were Alyson Hannigan who became Michelle Flaherty and had a more famous tag line: "...one time at band camp...", followed by "How I Met Your Mother" and David Boreanaz went from BtVS to AtS to Bones without a break.
Definitely SMG should put in an appearance at Comic Con and introduce herself to some of the generation that may not have watched the show because they were too young but follow the series in the comics.