Sharalayan / Farplane ("Mukou" / "The Other") -> Ala Mhigo ("Insurrection" / "Tempest") -> Doma / Beyond Eorzea ("Past the Border" / "Borders") -> Garlean Continental Empire ("Hangeki" / "Turn") -> ??????
A haphazard text-wall of a few of such mechanical content ideas: (Needless to say, even a small portion of them will be impossible without a larger dev staff.)
Late Heavensward adds three-dimensional combat (flying enemies that will actually fly and divebomb in ways that, while quick, can be queued against and melee attacked, player extended/supportable jumps, airship combat, etc.), the Stamina system (for dynamic movement, enhanced evasion, or knockback resistance), gambits / AI improvement, the mission system (making variably instanced combat occurring around the open world or in locations based heavily on the real open world a very real type of content for casual, midcore, and hardcore players alike, Hunt revisions, the Echo model (including live viewing and/or recording of freely spectate-able fights), massive increases to player training support, and partitioned fights for the DF (saving progress across phases to unlock latter phase-focused parties, bridging the gap between upper-core content and casual playtimes/playstyles.
Mukou drops PS3 support; expands certain maps; unifies the cities into a single main map instead of two halves while expanding their overall size and significance through enlarged guild halls, trade unions, and smaller in-city factions and groups, many of them with unique quest styles and interaction with other class trials; returns partly to and around 2.0 areas in addition to a new greater region, giving a fuller, deeper, and less disjointed sense of the world; gives a massive revision to the armory system, allowing cross-class experience gains and unlocking abilities (or more accurately ability types) from specific classes and instead giving them through cross-class experience gains based on the roles, techniques, and themes experienced (many abilities are now given through challenging but fun quests that train either the general idea behind the ability or the ability's use itself, which are accessed through sufficient related exp); combo system revsions; massive trait revisions; Aspects, a form of class specialization that creates very large differences in gameplay or even niche based on relatively small original differences (for instance, a BLM would have access to Soul of the Evoker, roughly traditional Fire burst - Ice regen, Soul of the Invoker (which focuses on spell mantles (replacing Surecast) to build up burstable combos and "spell shotguns" while balancing personal restoration and defense), and Elemental Heart (which follows a different mechanic per elemental "Flux", from the ramping Fire to opportunistic burst Frost to deadly utility Lightning stance)); new ways to bolster gear, such as through challenge runs and new trial and mission types (with different outcomes and challenges aligned to different elements); secondary stat revisions, gear scaling revisions; potentially stackable potency-based buffs and debuffs (following a mathmatical equilibrium between debilitation and effected mitigation, etc.), finite resistances, and a revised elemental wheel system that allows the stacking of elements onto enemies, allowing for instance BLMs to stack ice on an enemy to progressively heavy, bind, and even deep freeze it; AI and enmity revisions that makes the job of tanking more about distraction and thwarting enemy offensive efforts than simply meat-shielding; progressively-mitigate-able cleaves (damage reduced by mitigation done from the first person hit carries on, stacking, to each person behind; certain AoEs are immune to progressive reduction from most forms of mitigation); Spirit and the "Link" system (allowing additional effect from combination of one's own abilities or more often coordinated attacks between oneself and an ally); and potentially a Stagger system, wherein damage itself has a suppressive effect (previous suppressors having either individually tremendous or lengthened suppression or unlocking new suppression mechanics/pathways on the target) such that DPS now play a role in party survival. Oh, also adds more realistic running, mounted, and flight physics, even though flight is much more limited than in original Heavensward--oh, and stealth mechanics. ...And introduces the idea of NPC progressive AI companions, though primarily as tools for lore and character development at first. Sidenote: Classes are now worthwhile in roughly half of all content and slightly preferred in about a fifth. [The sometimes quirky, sometimes almost artsy, sometimes philosophical, sky's the limit, having huge fun even if without epeen gear to show for it expansion.]
Insurrection improves vehicular combat, adds multi-manned mechs, massively revises crafting, adds advanced item construction and open-world player-influenced salvage, partial world PvP, partial open world constructions, auto-generated landscape, multi-path and much deeper, more difficult MSQ, numerous character relationships and player-derived outcomes therefor, battle regimen improvements, improved field marking, Briefing system, dynamic enemy encampments and raid grounds, a vastly revised idea of what it means to be a "Raid"; revised range systems (snipers are now a thing even if not part of general combat), improved movement mechanics along the lines of both free-running and combat-climbing enemy mechs... [The heart-tearing, gritty as hell, complex as hell, pinnacle of the "mission" system expansion.] You can work for or alongside certain Garlean generals in this expansion. There are some rebels whose actions many of you simply cannot condone. It's a enthralling cluster****.
Borders brings in/furthers mounted combat, greatly improves exploration, faction building, allows much more significant player development over the real world, and more notable world PvP, more detailed companion building; further revises crafting (materials have actual effects rather than simply composing a level range, and recipes are now much more free-form (likewise, purchased items can now be modified by part, such as changing out the shoulderplates on a purchased chest piece) -- carried/equipped weight is now a factor; mount revisions (different mounts have different advantages rather than being merely glamour alternatives, and only one is taken out of the city with you at a given time); gives bag revisions, including saddlebags; primary stats revised to be usable across a greater range of classes and jobs, and gear is much less class-locked; gives further cross-class opportunities and the ability to create hybrid abilities of sorts; greater open world AI behavioral depth (packs, herds, and camps, often requiring far greater strategy to attack successfully or just non-fatally); more locally-differentiated lore and related mechanics. [The subtly guided exploration/sandbox expansion... power to the players while making badasses out of their variably traumatized selves]
Garlean expansion... [hands are cramping up, so ending here; kudos to those of you who could actually read this whole mess] adds espionage, even further mission planning, improved communication systems, improved minion systems; custom hero classes/advanced jobs...[The broad or top-of-the-world view, thoughtful, or just... 'I'm really turning into a badass' expansion]
Last note: Each expansion's content should remain significant / run in parallel to some degree into the other, while each expansion kind of takes a turn of theme by its half-way or final patch, almost to where it can be called a different game. Farplane, for instance ties together 4.0 and 5.0, while Solace runs from and partly parallel to the end of 5.0 and the start of 6.0. Similarly, any mechanical revisions will take the time where possible to carry back into prior content (e.g. the crafting revisions in across all three expansions listed here going back to 2.0 recipes or possible products).