I've kind of got another question, pertaining to the whole "who dips their toe in the FullDive pool next" question)... how much does anyone know about what caused the SAO incident?
Argus has utterly crashed out of existence, and Kayaba's own personal stock should be in the sewer because he's [The SAO Guy], but he hasn't gone on TV and given a big supervillain speech with maniacal cackles or anything, so he's presumably been cleared of deliberate wrongdoing, turned over all the (official) documentation for SAO/Cardinal/the NervGear, and retired to his mountain cabin after an "emotional breakdown". There he can chill in private with a ton of gutted-and-networked Playstation 6s serving as a supercomputer (and heating system), his location legally redacted to protect against death threats from distraught relatives. Unlike canon-Kayaba, he presumably intends to survive Log-Out and guide the players to revealing magecraft - so just up-and-disappearing in the middle of the investigation would be counterproductive.
So what does everyone else know about the SAO incident? Kayaba's professing horror and ignorance, and his coworkers should be clueless because of hypnotism - except Koujiro Rinko, who's worked out that something's fucky and is playing out a techno-thriller tragic romance movie trying to track Kayaba down - so as far as investigators can tell, the actual designers of the game either aren't at deliberate fault or are all part of an incredibly disciplined and bizarre conspiracy.
Which means that someone outside the design team managed to a) prevent players from logging out within the game, and b) prevent manual logout by modifying the NervGear schematics to turn its battery into a manual-logout-triggered bomb, getting it past QA in the process. Which is, holy shit, going to stump them. How would you even start investigating that? Try to track down groups capable of hacking into the Argus servers and modifying fundamental and highly-secret files of both game code and manufacture blueprints? It can't be terrorist action, because no-one claiming responsibility, so is it industrial espionage gone wrong? Some kind of horrifically unlikely bug that slipped through all the checks? The Chinese PLA's cyberwarfare units going extreme black hat?
...man, you can bet the third possibility is what sees funding poured onto Kikuoka Seijirou's cybersecurity unit.
(actually, is the whole "battery microwaves your brain" claim even true, in this setting, or is that a line of bullshit Kayaba fed the players to conceal the effects of a fatal geass hidden in the terms and conditions?)
[hr]
Alright, so to put forward my own game concept...
Sports simulators are an obvious choice. Extreme sports are the best choice, because they remove the issues of venue, equipment and injury - skiing, skateboarding, skydiving, surfboarding, waterskiing, demolition derby, etc. Hell, I'd expect a paintballing game as an alternative to "real" shooting games, promoted to parents as less psychologically dangerous. Wrestling/UFC could work too, especially since you're removing the need for constant conditioning or recuperation. Expect a MUGEN-style mashup of dozens of bland Olympic-rules fighting sport games into a faux-Street Fighter, before they just get around to that for real.
A form of sports simulation that bears specific mention... hunting games. As it stands, hunting games are mostly just first-person shooters wrapped up in survivalist trappings, but in FullDive it can take on new life. Different creatures, climates, technology levels... The AIs represent different animal behaviour patterns, which adapt to player actions and the dynamic climate, taken from (and in turn, funding) actual studies of wildlife behaviour - the bigger games probably have a branding partnership with wildlife reserves or preservation groups, both for that purpose and for the PR (look see we're supporting animals so it's not gross). Stalking a tiger through the jungle with nothing but a combat knife, or riding in the back of a jeep and taking down impala with a rifle, or enjoying the Dinosaur Hunt DLC and going after a stegosaurus with a shotgun. There's speculation about a Poacher Hunt Pack that introduces human enemies, but the next update currently on the horizon is a Wildlife Park Expansion where animals you capture instead of killing can be sent to live in a reserve you design and maintain for online visitors.
Those are both fairly generic ideas, though.
[hr]
To push specific ideas for MMOs...
Well, to spring off hunting games into bankable IP, I'll suggest Grimm Outlook. The game world is a vast, vaguely-European forest, dotted with towns, worn-down castles, mountain church-communities and so on. These [Grimm Outlooks] remain stable, but the forest around them is a dark and shifting place, complete with monsters, lairs, and dynamic-dramatic weather/terrain governed by the much-hyped, much-licensed [Sturm Und Drang] specialist AI director. The theme here is fairy tales - the players are [Grimm Reapers], a guild of fairy-hunters who track down and destroy fairy-tale inspired monsters created by the Fair Folk.
The game focuses on the hunt rather than combat itself, and so steals a lot of mechanics from the hunting games described above - tracking, stalking, camouflage, identifying spore and marks and habitats, adapting to weather, survival in the wilderness, and so on. Players have a [Lorebook] that they fill up by encountering and conquering threats, and can trade information to other players by expending valuable [Ink Vials]. An example quest might involve coming across a small village community and have to work out which of them is the [Big Bad Wolf] by comparing their alibis or performing a stakeout on an in-game full moon - or filtering through your gathered lore to work out the signs or tracks to look for - then taking it down.
The game presents a system-efficiency measure as a major selling gameplay point - everything outside the immediate area of a [Grimm Outlook] is a series of procedurally-generated instances in the [Fairy Forest]. A complex series of tags (and clever use of Sturm Und Drang) governs a largely seamless random transition from one instance to another, but this journey is randomized, with no guarantee that walking down in the same direction will take you to the same place. Your [Lorebook] records maps of an area while you're in it, but there's no minimap, and areas are discarded and replaced on a fairly regular basis. Instead, you need to use [Breadcrumbs] to mark your way back to the stable trail, or establish a [Campsite] to anchor an area so you can return and re-explore it.
While the early game is dominated by straightforward questing - encounter plot hook, pursue it into the [Dark Forest], fill out Lorebook, get crafting materials, level up skills - a "plot" kicks off once you form a [Band] of Reapers, and the game develops a [Fairy Godparent] based on your favoured enemies etc. This is effectively an AI Director "nemesis" that tries to produce a "story" for you to follow through the instances of the [Fairy Forest], with [Grimm Groves] serving as persistent dungeons (albeit not necessarily a literal dungeon crawl) that advance their group's story. The developers recently announced they'll be moving outside of Eurofae tropes, with slight tweaks to overall visual design to retain a coherent style rather than a sudden leap from Ogres to Oni.
[hr]
The other idea would be Dead Drop, a game where you play as a spy belonging to one of a number of espionage agencies, performing missions, running interference, sabotaging and assassinating key targets, providing false information, and so on, in a somewhat fictionalized version of your own local country. VR Hitman, but with a focus on group play - solo is still doable, but an AI Support Team doesn't have as good stats, and some people claim the random number generator is biased against them - and a strong social aspect. Can your [Deception] rank + Facial Recognition Cues + fabricated [Alibi Bonus] beat the AI's Lie Detection Rating?
Initially, you chose one of a handful of agencies to belong to, which were run by motiveless AI who worked in collusion to produce a believable conflict in which no-one ever achieved actual victory - a 1984-esque shadow war for the entertainment of the agents involved. The issue being that the AI developed "emergent motives" for their respective agencies that justified their actions in-game, resulting in unplanned-for splinter-groups and mergers and takeovers and so on whenever an "out of character" action was called for. This turned out to add a lot of layers of fun, and it's not like the [Agency] AIs are developing actual personalities and ideals, right? So the devs ran with it, and now the tutorial mission assigns you to an Agency appropriate to your chosen tactics, moral choices and stated political affiliation, until you're advanced enough to defect.
A particularly interesting part of this game is its ARG aspect - namely, by linking Dead Drop in with your social media and smart-tech GPS, you receive non-regular instructions to perform some action in the virtual space - go to your local shopping centre and add a particular VR tag to the Dead Drop app, or send a message to a particular number at a certain time, or find a car with a certain numberplate and photograph it for the app, or form a flashmob and tweet about it under a numbered DeadDrop hashtag - that are rewarded with in-game benefits you'd normally need to drop cash for. The idea being that you're helping to fight the secret fight even in the "real world". This aspect of the game is locked for ages 18+ for obvious reasons.
(yes, this is just SPOOKS from Halting State – where players are being used to perform seemingly innocent espionage legwork – with an AI twist)