It's also probably worth mentioning that Leafeon and Glaceon are in similar situations - Debuted in the same generation, are only a duo and not a trio so there's less resources needed to make the dynamic work - but Espeon and especially Umbreon seem to be more popular overall. Sylveon is also pretty darn popular, but it's not part of a set of Eeveelutions. You could easily spin something out of Eevee evolving into Sylveon to still cover the evolution gimmick, but I'm not sure if it covers the full range of options. And like I mentioned before, the Kanto trio are arguably the most iconic set, but since there's three of them, it might get a bit messy trying to cover the full range of options.