I've been doing some testing, and it appears that we can do what we want by manipulating global and local priorities while Autosort is still enabled. Here is a possible scheme, open for debate:
0.1 This extension to Vortex is intended to facilitate manual sorting of plugin load order in Bethesda games (Skyrim, SSE, FO4 and so on).
0.2 It will only add additional functions, not modify existing ones.
0.3 The user interface will conform as nearly as possible to the existing Vortex style, so that there is a minimal amount extra for the user to learn, to be able to use it.
1. Added Features
1.1 Plugins can be assigned to conceptual groups.
1.1.1 Plugin groups are distinct from Mod categories but a broadly similar style of interface is used to create and edit them.
1.1.2 A standard set of groups is available via a button or the user can create alternative ones.
1.1.3 The standard set of groups is the one developed by the XBox community.
1.1.4 When created or edited, each plugin group is associated with a Global Priority in the range -10 to -80.
1.1.5 The un-named default group corresponds to Global Priority 0.
1.1.6 When a plugin is assigned to a group, it is automatically given the global priority for that group.
1.1.7 Assigning a group to a plugin has no immediate effect other than to set its global priority.
1.2. Groups can optionally be displayed in an extra column in the plugin window
1.2.1 The column can be toggled on and off using an extra entry in the existing settings panel.
1.2.2 The extra column defaults to 'Off'.
1.2.3 The list of plugins as viewed in the window can be sorted up or down by group.
1.3 When load order is Autosorted, plugins in the same group will be kept together automatically.
1.3.1 Sorting by group is tantamount to sorting by Global Priority, which over-rides rules.
1.3.2 This does not in itself require any additional coding
1.4 Plugins can be manually moved up or down in the load order within their groups.
1.4.1 This is accomplished by changing their local priorities.
1.4.2 Moving a selected plugin up or down one or more places is done by drag-and-drop and/or by using the keyboard up and down arrow keys.
1.4.3 Moving a plugin in this way results in its local priority being set to a value intermediate between the ones of the plugins immediately above and below its target position.
1.4.4 This may necessitate bumping the local priorities of other plugins above and below the target position if there is no suitable gap.
1.4.5 Local priorities will be assigned in the range -10 to -80.
1.4.6 It is allowable for more than one plugin in a group to have the same local priority.
1.4.5 When load order is autosorted, plugins in the same group (i.e. global priority) are automatically ordered by local priority.
1.4.6 This feature of Autosort does not in itself require any additional coding.
Updated 2018-02-15 to reflect changes in Vortex 0.13
Edited by OldMansBeard, 15 February 2018 - 08:09 AM.