Thanks for the thoughts on "duration attacks". I agree that their interaction with reactions could be confusing. That mostly means more rule book text, but probably does weaken the chances of something like that ever happening. In regards to the cost increaser specifically, it could be fixed to prevent broken situations... but the fact Cutpurse started that way and changed on publisher request pretty much completely rules the idea out.
However, there might be other ways to get opponent hand effects without being an attack exactly - kind of like Smugglers, you could get some benefit from opponent turns with in an play card ("while this is in play, gain a coin token when an opponent buys a card") or messes with people in a "neutral" way and thus isn't considered an attack ("while this is on play, whenever one would draw a card, gain an action instead; whenever one would gain an action, draw a card instead"). A reaction-attack (which could be responded to by another reaction to block the attack) might be another way to make the idea work ("when a player gains a card you may discard this. If you do, they gain a curse.)
There could also be "altering effects" on your own hand only ("when this is in play, +1$ for each action card played. I doubt any of these specific cars will happen, bit I would not at all be surprised to see some sort of new "lingering effect" mechanism introduced - either through "use later" cards or cards with effects that aren't completely resolved immediately or some other way I haven't imagined. While durations and con tokens do this to some degree, I feel there is a lot of room left in that space.