Forever is a long time - A new mechanic

I've been working on a keyword lately that needs some help.  Let's call him Eternal.

I have two wordings that could be plausible, the first making the keyword an actual keyword, the second making it merely an ability word.

 

To me, the biggest problem with this mechanic is figuring out what the additional cost is going to be.  I have a few suggestions, but any others would be appreciated.  The "exile cards from your hand" cost seems a bit steep, but if it's kept to one or two, it's probably quite powerful. The first alternative is simple discard.  (Just a note: all the alternates will be with the word Eternal used as a keyword.)

 The thing I like about discard is that it feeds the mechanic.  If you discard a card with Eternal, you can cast it from your graveyard later by discarding something else.  I feel the cost of exiling is much steeper than discarding; especially if the cards can be cast from the graveyard.  These can probably have an Eternal cost ranging anywhere from one to three.

Another option is exiling cards in your graveyard.  I don't like this one for a few reasons;  the first being that it seems too similar to the Innstrad Skaab mechanic. The second: it's counter-productive to exile cards that you might want to play later.  It would force players to use less Eternal cards.  This iteration would best be served in a graveyard heavy block with an Eternal cost of two to six.

This version seems good, as you can trade out creatures in play with ones in your graveyard if the Eternal cost is only one or two.  I can see a huge black monstrosity having an Eternal cost of five with this version.  Like an 8/8 for 1BB that can only be cast from your graveyard. Yeah!

 I'd like some input on which version you all think is the best, and maybe some examples of your own to illustrate how awesome we can make this mechanic.  As always, thanks for reading, and feel free to comment on the blog itself.  ( I removed the "prove you're human" baloney.)