I'm going to use a word you don't see on the forums very often; buff!
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Before I get stuck in, I'll say that no matter what piracy/interdiction mechanics are implemented you'll always have the risk of those players or NPCs who's sole aim in life is to murder. No matter how you try to improve piracy that will always be true and anything which is implemented in favour of the 'proper' pirate stands to be abused/gained from by the more psychopathic players. Those advantages will also be gained by bounty-hunters, so it should balance out (but will leave traders more vulnerable).
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However, in Elite I think that's a risk that all traders should have to bear - whether in solo, group or open mode.
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I already agree with the more obvious suggestions in the thread: Improved system security responses, harsher in-game criminality (including getting rid of the ludicrous ability to pay off an assault or murder charge), improve the whole black market economy, vast improvement to cargo limpets.
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Buff mass-lock/FSD delay: It is far, far too easy to escape from interdictions via submit/re-start frame shift drive. Pirates don't get the opportunity to demonstrate their superiority in a controlled fashion (and thus negotiate terms of cargo surrender). Same for bounty-hunters.
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(How about a weapon which introduces a temporary gravity field around it's target, stopping them from launching into super-cruise?)
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Expand non-lethal weapon choice: Something that temporarily disables or degrades ship systems would be of huge benefit to both parties. Pirates might get more time to use limpets, defender might cause enough confusion to be able to escape. Grapple guns (pulling the target closer)? EMP Mines (area of effect detonation after a certain time period)? You get the idea.
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Improve the ability for bystanders to intercede: I'd like a whacking great "crime reported" navigation mark to appear in super-cruise if I'm nearby, which I can navigate to and drop-out at. (I appreciate the technical issues with this - given the networking model used, but still...) Traders shouldn't feel alone - they should feel protected by the police, bounty hunters and potentially armed merchantmen.
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Improve comms as well. Would it be too hard to implement a communication wheel which allows pre-determined messages to be sent to players or NPCs (so the NPC could react accordingly too)?
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Just some thoughts from a die-hard trader...