Putting him in the open world wouldn't change anything, it would be posted within minutes. And even more lazy people would be asking for an app to buy things from him because even the simplest tasks are too much for their lazy, soft, a.d.d. , pampered behinds to do. Putting him at the end of a mission would be even more complicated. First off you would have to have your coins and motes with you, plus if you are upgrading armor or weapons you need them with you. What happens if you can't find someone to do the nightfall with you or you are doing the weekly heroic and the matchmaking sucks and you have constant afk and/or deaths. In short, it's a horrible idea that doesn't fix anything only has possibilities to make it worse. Xur is fine the way he is. The only thing I would change is instead of coming once a week for 48 hours he should come for two non-sequential days like Wednesday and Saturday for just 24 hours each. Change up inventory each time. Would make you think if you wanted to spend your coins earlier or wait for the weekend. But he is fine as is.