Looking at previous additions / new releases / in-game events which caused sustained increases in general activity:Aaaand after few days this online peak disappear.
Sadly, I don't think that adding new shiny, shiny content each week is possible
- initial release of Horizons (several months)
- initial Thargoid attacks in 2.4 (persistent increase, kept going until masked by later Beyond releases)
- engineering rework in 3.0 (a few months)
- mining/exploration rework in 3.3 (several months)
- Fleet Carriers leadup/release (several months)
- start of NMLA/Azimuth/other storylines and/or Epic giveaway and/or pre-Odyssey hype (persistent increase until Odyssey release) [The storylines did have fresh weekly content, as an exception to the general nature]
- Colonia Bridge CGs (a few months)
- introduction of Thargoid war (several months)
So based on that the big things seem to be:
- new features that allow new ways of experiencing existing content (mining, exploration, fleet carriers)
- new content which generates fresh weekly scenarios (either mostly automatic like the Thargoids or mostly manual like the NMLA story)