Content, not just endgame but also more use of existing zones for all levels including zones that don't have a lot going on in them. More story about existing characters and places.
More Task Forces/Trials/Story Arcs for group content. Additional archetype/origin-specific solo story content.
I would be fine paying for a subscription to support the game, regardless of how much that subscription costs (cheap or on par with modern games) as long as content and AT/character content was being added regularly.
Microtransactions are ok, as long as it never becomes pay-to-win/pay-to-enjoy. Aesthetic/cosmetic/convenience microtransactions, but never pay to get inf/powers/level 50s, etc.
