It's not worth having ideas. Anything I come up with would be too different anyway. To wit:
<serious = "off">
Leave SoD as it is and start something new but related from scratch. Try to work from lessons learned and avoid long-standing issues before they have years to become entrenched.
Example: automated events, like the War. Events are great in theory but always come up against the issue that low levels are flat out incapable of contributing to anything that's worth higher level players being involved in. There's no fundamental reason that large level differences need to make all attacks miss/get resisted. Take that out. Maybe change heals to work in the vein of "heals for 75 or 5%, whichever is higher" to allow lowbie healers to contribute relatively small but non-trivial amounts when high level players are taking the lead. (Probably crib % mana costs for heals to make that non-exploitable and fix the "high end healers have infinite mana" issue at the same time.)
In the future, Kaezul's forces have taken over almost all of Dalaya. Only <city> remains free, and members of all races have fled there, but even it is in jeopardy. The skies are an endless sea of grey -- the city is always darkened, remnants of Kaezul's twisted magic. Raids on the city are commonplace; everyone knows it's only a matter time before a proper assault is mounted. The guilds that once defended the city are in tatters, splintered after recent failed retaliatory campaigns. Few people openly wander the streets. The economy is ruined -- even beetle eyes are considered worthless. Shops that once sold fine goods now offer only rations and rumors.
In the midst of all this, the remaining Guild Masters -- many from regions now under occupation -- are working in secret to recruit and train a new resistance to strike back at the Kaezulians any way they can: attacking patrols, raiding outposts and caravans, liberating cities; perhaps even fighting wild powers not even the Kaezulians have been able to contain...
* Centralized hub zone where new players all start, so they can find people to group up with as easily as possible, even when low level players become scarce. Lots of quests can go there, making all of them more accessible and making working on quests more attractive as a dev (sucks making a quest only a handful of people do for years). Easy theme for quests and lore too: war tensions, rising in ranks, missions, etc. Zone difficulty can fan out naturally from the starting city zone. Lots of zones can just be reimagined from current SoD versions.
* Rewards: change reward structures to allow more room to grow. Give items tons of aug slots. Maybe have aug slots segregated by source: tradeskill-sourced-only aug type, grinding/dungeon-sourced aug type (both zone specific drops and wide-ranging/global random rares), augs type that only comes from events and quests, etc. Main slot items would be mainly from raids and tradeskills and maybe droppables, as now.
* Non-farming, non-grinding tradeskills. Tradeskill items are made out of random rare wide-ranging/global drops. No combine failures, no skill grinding; creating items mainly gated by skill requirements and scarcity of dropped supplies. Non-"trivial" combines always increase skill. Tradeskilled gear should never become completely irrelevant; very rare drops from super high-end zones make super high-end gear.
* Change how money is obtained. No more market for rat fur. Using Kaezulian currency considered treasonous. Would need to consult someone whose eyes don't glaze over thinking about "virtual economies" to work out a specific idea, but in general: non-farmable. Maybe some kind of stipend depending on quests/"rank"/level. Or something really out there like a strict limitation on how much money exists in game at any one time, increasing at a slow rate.
* Wackier things like changing how leveling up works, how damage and stats work, etc. Redesigning spells and abilities from the ground up. Maybe giving classes more leeway to fill alternate roles while still preserving their expected specializations. Maybe make some lower level zones more dungeon crawl-y/raid-like, why not. Maybe keep the level cap extremely low (e.g.: 10 to start) while work is done doing all the spell and skill redesigning, content creation, etc. Keep people from getting super far ahead off the bat while still letting them grind augs and tradeskill items.
* Make some stuff instanced. Can keep open world mini-events like tmaps, but realize that Wiz's old justification of being able to stumble onto stuff happening and see that the world is "alive" doesn't really apply to out-of-the-way dedicated raid zones.
* Make stuff like meditating, buffing time, and downtime in general much shorter. Maybe minimize long-term buffs altogether. Raise base movespeed.