I think these are all great choices for themes of precons, but I personally am not interested in any of them.
I feel like everyone should build a colourless deck at least once (I've built many and love them all), and this will probably be a decent foundation for people to build from.
Enchantress has never interested me personally (most of you know of my indifference to GW decks) but again, a common archetype that appeals to many so always a good choice for a precon and in the superior choice of colours (any deck with black in is more appealing to me than any deck without black in).
Same with Superfriends. Not my thing, but people are in to it, and Guff is cool and has a funny name (for those who don't know, in the UK [and probably other places too], guff is a slang word for a fart).
Slivers is actually a really cool theme for a precon. If I were to pick one of these up, it would probably be the slivers one. I feel like slivers are somewhat less of a "kill-on-sight" kind of deck than they used to be. Sure, they're powerful, but like all tribal/creature based decks, they're weak to boardwipes and options for mass removal, especially mass
exile, keep getting better and more varied, so I feel like this will be just a mid level tribal deck with cool synergies and interesting interactions, and that sounds like a good time to me.