What about "When CARDNAME enters the battlefield, sacrifice it unless you return a creature you control to its owner's hand." That would be a very synergistic drawback.
I like the card. Obviously you can do what you want, but I'm pretty sure wizards nowadays try to avoid things that are even marginally strictly better than basic lands. It's a shame, but I'd expect to see this paired with a very small drawback (smaller than "pay 1 life" since that's what dual fetchlands pay for functionality which is normally stronger than this), maybe "legendary" (most of wizards' slighty-better lands are legendary, I think?) or "this comes into play tapped unless [very easy condition]" or "when this comes into play, your opponent gets [something small]"
That would be exceedingly strong. I suppose it's comparable to Fireshrieker, but that may be a bit highly-powered like many Mirrodin-block equipment. Compare Sunhome, Fortress of the Legion.
Think the Marianas Trench kind of darkness and crushing pressure. Whilst it is 'strictly better' than an Island in some ways - you can get or out of it the turn you play it - getting has a fairly hefty cost attached. (Was originally sacrifice a creature)
Power and Toughness switching pops up in both colors. Spell redirection does too, as in "target creature you control becomes the target of target spell with a single target". Target target target target. You could also do something like "Target creature you control is a 3/1 red and blue elemental until end of turn."
I think I may mix up the two suggestions and have it ETB tapped unless you return a creature you control to your hand
What about "When CARDNAME enters the battlefield, sacrifice it unless you return a creature you control to its owner's hand." That would be a very synergistic drawback.
I like the card. Obviously you can do what you want, but I'm pretty sure wizards nowadays try to avoid things that are even marginally strictly better than basic lands. It's a shame, but I'd expect to see this paired with a very small drawback (smaller than "pay 1 life" since that's what dual fetchlands pay for functionality which is normally stronger than this), maybe "legendary" (most of wizards' slighty-better lands are legendary, I think?) or "this comes into play tapped unless [very easy condition]" or "when this comes into play, your opponent gets [something small]"
That would be exceedingly strong. I suppose it's comparable to Fireshrieker, but that may be a bit highly-powered like many Mirrodin-block equipment. Compare Sunhome, Fortress of the Legion.
I'd lean more towards first strike.
Cromat had a similar problem. It ended up using flying for this colour-pair.
But I also like "switch target creature's power or toughness".
"Gets +2/+2" would fit best in those colours. It'd be like an artifact version of Nantuko Disciple or Kjeldoran Elite Guard.
Think the Marianas Trench kind of darkness and crushing pressure.
or 
out of it the turn you play it - getting 
has a fairly hefty cost attached. (Was originally sacrifice a creature)
Whilst it is 'strictly better' than an Island in some ways - you can get
Target creature is a 0/4 treefolk until end of turn? Sort of... :)
Power and Toughness switching pops up in both colors. Spell redirection does too, as in "target creature you control becomes the target of target spell with a single target". Target target target target. You could also do something like "Target creature you control is a 3/1 red and blue elemental until end of turn."
Was going to have Flying (Red has Dragons after all) but not too sure that is best fit for
but what else is there?
The weak one of the "cycle" not sure what other keyword besides Reach is both Green & White that would be suitable
considered having this give Double Strike but that might be too strong.