It is always a bad idea in the development cycle of a software product to change basic default behaviour to the opposite of what has been established over years. You can of course allow things in a certain version that were not allowed yet in the previous ones, but only after an opt-in for this behaviour change. So add a new check box "Automatically activate newly installed plugins" in the basic WP settings which is off by default and do not change the behaviour of installing plugins to something else people might now expect. And maybe add a hook that allows to opt-in by calling it. Or add a define('WP_ACTIVATENEWLYINSTALLEDPLUGINS', true); for the wp-config.php. Or, like already proposed, add an "Install and avtivate" button on the plugin install page. But again, leave the default behaviour as it is now.