The best thing you can do is to create a series of creatures (one for each element), much like waterzx's Wizard series (who is, by the way, using Emerald Tiger's art for his cards). We could have something like "Archmage of Aether", "Archmage of Air", "Archmage of Darkness", "Archmage of Death", "Archmage of Earth", "Archmage of Entropy", "Archmage of Fire", "Archmage of Gravity", "Archmage of Life", "Archmage of Light", "Archmage of Time", "Archmage of Water", each one with the ability "Archmage of X alternatively becomes the Archmage of your mark and the Archmage of X each turn.". They may also have different statistics and/or different activated abilities. However:
1) When the card changes from Creature A to Creature B, will it transfer the damage it has got into its new morph? If yes, then what happens if Creature B has less max hp from Creature A? Does it die instantly, or it remains alive at 0 hp?
2) If the card gets buffed (let's say by Blessing) in Creature A state and then changes into Creature B, is it going to transfer the buff to its new form?
3) Are poison counters, delay, frozen etc. transfered between the alternative card's creatures or not?