I suggest the following:
Elite mission Double Kunaxai and Urgoz drops (so 6 people get greens), have very select monsters (Aspects of Kunaxai, maybe add an aspect of Urgoz), that drop one green at a rare drop rate.
Award a few amber or jades for a Quarry/Aspenwood weekend, to increase activity in those areas.
As posted as an idea of a sorts in prior posts, a "Threat to all of the Gods of Tyria" UW/Fow, where favor is not needed to enter UW and Fow, but instead of double ectos and shards, you almost double the monsters in the areas.
Have guild alliance weekends, where one weekend, rating is farmed by the alliance in gvg, and awards are given to the top 16 alliances (can be limited edition cape colors, some pve rewards such as a customized only weapon/offhand award). In other weekends, alliances compete for Kurzick/Luxon faction, where the top faction farming alliances for the weekend get similar customized only rewards, or even gold prizes, or rare minipets, something like that.
A titles weekend, where select titles get special treatment - drunkard title requires less ale, cartographer title has special areas reopened where they couldn't access (Tyrians exploring locked Canthan areas), luck title (bring back tickets for a weekend) champion title (applies to high level gvgs where you beat a 1300+ rated guild with double the reward, can be done even during an offseason to get rid of extreme rating farming setups), skill hunter title (you can cap any elite despite primary/secondary class at the time), treasure hunter title (double points), wisdom title (double points), alliance titles (double faction actually donated towards titles). - All to encourage pvp and pve in a weekend, and help restore, at least for a little while, the increasing lost sense of community to GW because of people leaving, quitting, idling because of inactivity, etc.
Run "experimental mods" weekend, where features such as LFG tombs and gvg pug guild boards are added to the game, even in game voice chat, for a little while, and use it as a way to get feedback on different features that may/may not be added to the game, before it is even fully implemented. Logistically hard to implement, but it may be possible for several features, as who knows, it may be done but waiting for the next chapter.
Other things for pve: rare material drop increase (apart from ectos and shards, that will probably create mass GW market crashing, uber market speculation = chinese farmers smiling as they sell more and more in game gold), have occasional drops for materials like deldrimor steel, bolts of damask, etc..
New greens weekends (to become permanent), where some old, unpopular greens (Sorrow's Furnace, Tombs pve drops) are replaced with new skins or by entirely new greens, or can be additional possible drops to their otherwise traditional green drops.
Pvp suggestions: Create a 6v6 arena.
Restore 6v6 tombs, double the fame, but bring back the old tombs layout/modded layout (2 team UW, 6 team Burial Mounds, 3 team broken tower, 6 team scarred earth, rest as the same layout, with a 5 team HoH each time).
Eh, since these are suggestions basically, I also suggest reducing the price of Prophecies/Factions after a given period of time, so that new players to the new chapters can buy the old chapters (I highly doubt many will buy prophecies at 49.95 at the online store if Chapter 5 comes out
, but may at say, 19.95).