It would be really great if they came out with a custom leader option where you could create your own leader. You could still have the limitations in place that still require you to have three types of infantry, three types of air units, and three types of vehicles (within reason), but you could select which three types from all the available DLC content you own. You could also select which leader powers your custom leader would have (again within reason) all of which could be played in a custom skirmish mode that would be used for custom leaders, or have a toggle switch that says allow custom leaders.
Cool idea! Not feasible tho
really the only way this would work is if you were given a point limit to your customisation, and only allowing you to select from pre done leader abilities from the current leaders and as well as already unique units for the current leader (example would be to have atrioxs dying breath with rage from arbiter being used on a hunters captain) however this will probably never happen since people would just pick the best leader powers and unique units forcing a single custom leader loadout (and 5000+ videos on “best custom leader for halo wars 2 guide” videos to appear on youtube ugh) as the meta