If you look carefully at the base stats of the Eeveelutions, you'll notice they all use the same stats in various place:
Vaporeon: 130 HP/65 Att/60 Def/65 Speed/110 Sp Att/95 Sp Def
Jolteon: 65 HP/65 Att/60 Def/130 Speed/110 Sp Att/95 Sp Def
Flareon: 65 HP/130 Att/60 Def/65 Speed/95 Sp Att/110 Sp Def
Espeon: 65 HP/65 Att/60 Def/110 Speed/130 Sp Att/95 Sp Def
Umbreon: 95 HP/65 Att/110 Def/65 Speed/60 Sp Att/130 Sp Def
So, if they did add a sixth Eeveelution, it would have to be one with 130 in Def, since all the others use 130 in the other five stats. This has led quite a few people to suspect the sixth Eeveelution to be Steel type, since they tend to have high Def. By the law of averages, it could go something close to this for base stats:
???: 110 HP/95 Att/130 Def/60 Speed/65 Sp Att/65 Sp Def
which would make the stats reasonably tankish, having an above average physical set with below-average special and speed, fitting the set up of most Steel types. As for the name, I have not a clue, apart from hazarding a guess it will end in -eon.
This is all speculation, but it makes sense considering the rest of the Eeveelutions.