Roglic has potential, certainly, but in terms of legendary status, he is far from Pantani, Contador, and Froome. He's also far below Nibali. His own teammate, Dumoulin, is ahead of him with a legendary post-poop performance, Giro victory, and 2/2 Giro/Tour against true legend Froome, among others.
In regard to charisma, I'm guessing Roglic and Froome are, basically, introverts, whereas Pantani, Contador, Alaphilippe, etc. are extroverts. While this isn't always true, I think introverts have a harder time building a compelling persona off the bike because they don't thrive on the energy and interactions with the press, etc. Unlike, say, an Armstrong, who still revels in his own myth-making on the Move.
Side note: I just read Froome's book and he talks a bit about his lack of charisma and stifling, boring politeness, which is the exact opposite of rock n roll. I won't get into the clinic questions his book raised (no particular explanation for sudden jump in form from Tour of Poland to Vuelta in 2011), but if you are awed by Roglic's rise from where he started, you should give Froome more credit. His developmental days riding for Kenya were a joke compared to 99.99% of the peloton. He also pulls no punches in that book, giving the straight truth on his relationship with Wiggins, Brailsford, etc. I didn't appreciate his disrespect for my man Contador, but I did come to appreciate him more throughout the book. Interesting perspective on the tactics he used and his own perception of the Vuelta 2011, Tour 2012, and Tour 2013, stage by stage. /digression