With the change to the Aura wording for local enchantments, a quick glance at the comp rules shows that Wizards has seemingly made it clear that enchanting players can and likely will happen. So I figured, why not explore this now?
The first thing I told myself about enchanting players is that while many global enchantments only affect one player and could be easily made into enchant players, that's not really interesting or new at all. For instance,
"Creatures enchanted player controls get +1/+1" or
"Whenever enchanted player discards a card, ~this~ deals 2 damage to that player" do indeed use enchant player well in a technical sense, but they definitely don't use it to its fullest. I thought, then, that trying to make something that could be cast on yourself or an opponent and have both choices be good in different situations should be the goal of my enchant player cards. This was the first thing I came up with:
Samba of Snakes

{G}{G}
Enchantment -- Aura
Enchant player
Whenever enchanted player plays a creature spell, put a 1/1 green Snake creature token into play.
The idea here is that you can play it on yourself as a more proactive effect, getting more Snakes the faster you play men, or you play it on the opponent and get more of a reactive effect, answering any creature your opponent plays with a token that can at least chump block. My idea for White, which I'm pretty happy with:
Cha-cha of Courage

{W}{W}
Enchantment -- Aura
Enchant player
Whenever a creature enchanted player controls deals combat damage, you gain 1 life.
Again, a proactive/reactive choice. Either your army gains you life when they enter combat, or your opponent's creatures essentially deal 1 less damage on a hit to you, or even give you some life if they get blocked. I really like how this is a card you could easily see in either a WW aggro or white-based control deck, two very opposite archetypes.
Blue was a little tougher to figure something out for, but this is my initial try:
Mambo of Mirroring


{U}
Enchantment -- Aura
Enchant player
Whenever enchanted player plays an instant or sorcery spell, you may pay


. If you do, put a copy of that spell onto the stack. You may choose new targets for the copy.
For you, it's a colored Mirari, but now you have the option of copying your opponent's spells instead, if you think they have some stronger effects (Tooth, anyone?). I'm not as happy with this as the others, because it seems a little uninspired, but it's at least a little interesting.
Red and Black, I couldn't really get anything too good. For Red I had the idea of 'Whenever enchanted player plays a land, deal 1 damage to target creature or player', which could be nice but again, feels a bit uninspired.
So, thoughts? Names are obviously placeholders. I'd kind of like for this to be a full cycle, so if anyone has ideas for Red or Black, I'd be quite interested.