...1. The campaign you'll be running will be at least 60 sessions long, more if you wish
....6. Also don't worry about getting players - let's say you have them and they want to play and will learn the game if they don't already know it.
....7. Let's say there's good online VTT support for whatever system you choose. I know, not realistic, but it's not a factor you need to consider if you would run this online.
Considering this hypothetical scenario has at least those 3, there's no way I'd run it with the 800 lb Gorilla driving the TTRPG biz jugernaut, which is D&D 5e. I'd definitely take this as an opportunity to use a less known system.
My 1st consideration would be rules that could run for the length of 60 sessions, let alone more.
That would stress a number of 10 levels only TTRPGs. I know based on the other criteria I could effectively run such a campaign with Adv OSE, but level ups would only come every 6th session at best. Which IME is pushing it a bit considering how important HP increases per level are for PC survivabilty, among other PC gains. Were I still running DCC, it would be in the same boat. So I'm thinking no.
13th Age is another system I like to run, but it's also 10 levels max. However it's designed to have PCs level up every 3 or 4 full heal-ups or 12 - 16 combat enconters. Due to 13A's narrative nature it presents a lot of opportunity for meaningful social and exploration encounters. As well, it has incremental advances so players can get say a feat, ability score bonus, HP increase, or background bonus after a session before they actually level. I love it's play around Icons and many of my players do too. So I'm thinkng yes to it, as long as I'm mindful to run less encounters that are combat oriented.
Savage Worlds is another system I really enjoy and it can be run with good results for any Fantasy genre. But SW by design is for shorter campaigns and advancements by default come quickly. Even using the slower advancement of every 3rd session, after 60 sessions that would put Heroes at Legendary+4 ranks. I personally don't enjoy GMing SW campaigns to Legendary+, because IME Heroes become omnipotent. The trick would be to use the older XP advancement in SW Deluxe edition and often only awarding 1 XP for encounter rewards. A GM can of course reward advancements only every 4th session, but my problem with that is that most experienced SW players I've run campaigns for expect faster advancement and begin to grumble when they're not getting it. For me XP rewards have been more effective, because the rate can vary such that PCs will sometimes advance after only 2 sessions that featured very challenging encounters and, at other times after 6 with a series of low challenges. That in general has proven to be more satifactory.
The Sword of Cepheus is another system I'd consider. It's a terrific TTRPG with a primarily targeted at Sword & Sorcery, but the latest edition also supports different Fantasy genres - even SciFi-Fantasy. It's a classic 2d6 system with balanced and enjoyable game play. I know it'd work, because it shares the same DNA as Mongoose Traveller 1e, where players take careers and after character creation a PC's deemed to be about as competent they'll become. It uses XP for advancement, and by default only 1 XP is awarded at adventure completion; 2 XP in rare circumstances. Meanwhile skills cost 10 XP to advance and can only be increased during downtime. There's an optional to give 1 XP after every session which would translate to 1 skill increase after 10 sessions. Depending upon the Fantasy genre I'm running, I'd probably use that option. So yes to using it.
Were I to use any D&D flavor, it'd be AD&D2e. I'm new to it, but I've read its options for slower progression. And from discussion with my DMs and fellow players, campaigns can run for years. Meanwhile it seems very flexible and able to handle many . Not to mention there's one heckuva larger library of publications avaialble for it.
Pendragon and the family of TTRPGs based around the Basic Roleplaying (BRP, Mythras, RQ6) are also able to handle all the criteria. So I might us any of them. And with VTT support, I go with my regular IRL table habit of having a personal computer with a pseudo-client up on the big screen. So that players that can't make the session can opt to play remotely while table players can view them.
