I don't think it will be only Soul Bound reward cards offered for ranked play. We still have SPS and Packs along with Merits and who knows what else in the future. If you mean Mix and Match kinda deal. Some Soul Bound Reward cards with Tradable that is an interesting idea but I'd personally prefer 100% of either or for any future Reward sets. Its certainly harder to win these days but the BETA/ALPHA cards are still strong enough in league play. Tournament play is another animal altogether.
RE: Splinterlands: Soul Bound Reward Cards