I'm very surprised that Thoughtseize was not included in Modern Masters.
That little black sorcery's price has been continually increasing. Does this mean that it will only continue to grow?
MM2 or M14/Theros.
Almost certainly not M14, given that R&D believes that 'new players,' the core set's target audience, don't like having drawbacks on cards. (Though I think they allow exceptions at Rare/Mythic, so it is possible, but quite unlikely.) It would be good to see in Theros since it does seem to be a stark omission from MM.