Browser is playing up & not letting me make flip cards properly. Anyway, I've done this before in my Snap non-set.
Much like duelmasters, there is no land and all creatures can either be used as creatures or as mana sources by playing them upside down into your resource row.

Thanks for fixing it Darkheart, much appreciated.

Yep, I think Magic might well have been a better game if it had been designed this way from the beginning.

To be honest, I think it's terrible. No offense Camruth, we're talking about the mechanic and not your card. I played Vs. and Duelmasters, and the idea sounded great at first, but then the predictability of duels starts to kick in. the same thing happens over and over again. Not to mention that bad cards never get played. Players only ever play their best cards.

There are ways of getting around this complication, like printing silver bullets and cards you want to play roughly half the time in normal games. But the cards you want to play half the time are hard to design, and the silver bullets are aggravating to play against. The entire design file gets filled with poor choices in the end, or the game becomes stale. If Magic wasn't a CCG, and you could build people's decks, then sure, you could craft a very good game. But in MtG, most players don't take the tools you give them and find out how they can make fun deck to play against... they take the tools and find out how they can consistently play their broken cards.

One way to get around this would be to only make common/uncommon creatures have mana sources attached so you had to play the "bad" cards to ensure you have a viable mana base.
Mind you, then you would have players complaining about being "forced" to play with the basic cards (which I personally have no issue with but can see how others might grumble).
Another option would be to have the "bad" creatures have the best mana sources (multicolour, etc) and have the better cards be weaker mana sources (maybe or pay to get for example)

I am definitely cool with having some cards be capable of being mana sources. I'm not too big on the game being super consistent, but it does lock out my other problem of people not playing with certain cards and always turning them face down.

Another option, which I appreciate, is World of Warcraft TCGs approach to this problem. You can turn any card face down for mana in that game, but there's one card type (Quests) that are much better when you do... they kind of got a morph thing going on, where you turn them over and draw a card if you pay (but they're still lands) or something like that. Again, on round 8, you will have 8 mana... which really bugs me, because many games play out the same way. But at least this time, you could be stuck with 'all land' and 'no land' hands, and in both scenarios, you can keep playing (pay mana to resolve quests, or turn 'real cards' into mana).

That being said, I don't think I've seen a game say "All cards must start as land, and must be flipped up into something else". That sounds like a great attack on the problem, and an interesting in-game challenge. Like to rush your opponent? Fine. But you've got to squelch your mana base while you do it. Want to play huge spell after huge spell? Fine. But you've got to find a way to stay alive for multiple turns without casting your big guns. Why am I not designing this game already?

Oh, one more game, while we're talking about maintaining resources. I rather like Battle Spirit's resource system, which... um... you know what... this would require a lot of writing, and I've already written an article when Battle Spirits came to America. If you really care, it's located at here, but I don't blame you if you'd prefer to skip past an old opinion of a game that's waffled in the American market (in contrast, that game is number 2 in Japan right now. Funny how practically no one recognizes it in North America.)

@jmg I'm very interested to hear, because I've often heard that the play-a-card-as-land system feels a lot smoother, but never been sure if the reduced variance would matter long-term.

It's a bit like comparing a faster leveling system in an MMO to World of Warcraft, or to an alternate RPG to Dungeons and Dragons. It's exciting, and you get what you want out of the game, so that's good. Unfortunately, it hurts the long-term playability of the game. For some games, that's not only acceptable, but encouraged. If the target audience is more 'board game crowd', like, let's say Game of Thrones the Living Card Game is, then this is a strong option (I should point out, that GoT's mana system is a very different animal, and probably closer to Magic. I'm just saying that they could have gone that way, and it would have been acceptable). When you're trying to design a game that's intended to captivate an audience who plays once a week for an indefinite time period, you want more variance in the core game. Unpredictable mana bases, games where you squeak out victory, or are smashed due to poor draws are necessary consequences. It also makes the games where everything lines up highly memorable... another necessity for high-traffic games.