Purity 3Exiles, Cost 3, Location
After each turn you discarded a card, your opponent must discard 1 card.
(Use this effect as early as possible to begin bleeding your opponent out of cards) or;
Purity 3Exiles, Cost 5, Exile Cost 2 (debatable), Unique Location
Whenever an exile cost is paid, gain excess resources next turn for the paid amount(s).
(This would allow crazy tempo turns following turns given up to pay exile costs, and present a situation for easy predictions and potential outplays by the opponent, not to mention any funny business possible)
I think Exiles as a faction has less potential as a split purity already, but few, if any, solid win conditions in a perfect purity deck. It's tough to find a place for them among the already existing factions, because most of the win conditions or goals in a deck scheme are covered or represented elsewhere. Thus they should have either something completely new as a keyword that works them into a winning or advantageous position (which they do but it's a poor boost and lackluster for skilled players), or, have many different cards that win for them in unique ways. Things that would be expected from the insane Exiles, are unexpected and can't be worked around normally.
Hence the cards of my design. Currently I cannot agree with the random nature of discarding for exiles, and I think it should be changed. If not, then the Exiles faction needs more blatantly strong cards or synergy, to make up for the unpredictable and implausible strategy in their normal decks following any form of discarding (excluding Yagron, but I strongly agree with his design [because there's no RNG!]).
Summary: Either give Exiles stronger synergy, or take away their randomness (least fun option), or make cards for them that present alternate but achievable win conditions. They will always have very little competitive play if they stay not only weak, but unpredictable to they player who is playing them! I mean seriously, look at Wholesale Slaughter -- I'd rather have one faction among all that is too strong rather than one that is too weak. A lot more can be said by me on this subject..
PS: This is all just my opinion