I already ask for this few times without success.
IPB should setup something so we stop sending tons of emails to invalid addresses. This is becoming a big concern to me and no, I don't want to pay to mandrill to have this feature, thanks,
Are there any plans to deal with non return members and users with disabled email accounts?
Since our communities send large volume of emails there should be a way to deactivate these accounts automatically for members with dead email addresses and members that don't visit the forum for a period of time.
This will ensure you have a clean domain, otherwise you will face problems with providers like gmail, hotmail or yahoo because you cant stop sending email to non existing accounts.
I don't think nginx/varnish cache would be very hard to implement. The greatest challenges would be purge content in nginx (varnish is easier) and implement a solution for view counters (varnish ESI is easy, but nginx isn't, maybe with ssi) for content like images, posts, downloads etc.
Would be needed to remove session for guests and this could leave to some unexpected behaviors that only IPB should care to handle.
I've tested a few scenarios but at this time I'm only using micro caching with nginx.
Regarding 3, 4 and 5 this is standard these days so there i hope we could see it in 4.0
About lazyload, this is a simple hack mainly for boards that publish huge amount of images like mine. It's not hard to implement and could give a push in rankings since you're not downloading images you don't see. I would like to see this has a default feature :) suite wide.