What do you want for stretch goals after 700k?
My own suggestions:
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£800k stretch goal: realistic blood and wound system like the one in Overgrowth (starts at 0:30): http://www.youtube.com/watch?v=6cXgvW_yHFQ&list=WL2h37Mp3UgQGjvo2FapQFWNTHLa6SAJ2y On top of that cloth/armor could be damaged accordingly which means that it could break locally and has to be repaired…
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£1M stretch goal: realistic weather system with real seasons like in AC 3 with snow/rain/fog. On top of that cold/warmth could matter as conditions you have to deal with (food, clothing)…
Feel free to discuss.