I feel like that are too many 'systems' already. I'd very much want to keep it simple. I am against instancing that's just a way to to avoid contested content, both on a group and raid level.
In addition to that, I think you could introduce enough ways to trigger yourself a raid target, and you dont need to limit it with 'x per week or anything either'. If anyone remembers Silithus you could solo/group farm stuff to summon a multi-group boss there. Put enough like that in alongside normally spawned bosses, so any guild that doesnt get a target on a raid night can go "ok guys the dragon is dead, lets break some seals to fight a demon lord instead". Reflavor as needed. Trigger stuff comes from group content players would do anyway for AA/coin/faction or maybe some solo stuff (Shawl and Ring in Velious being good examples).That way you already have another hook for people outside of raid hours, too.
Heck, with your game having weird planar intersections, it would even be easy to write into the lore! Instances would actually be mirror planes that collided with X or Y dungeon.Your guild needs to collect components to open them.However, loot that's gotten from there can only remain corporeal when worn (Hence the BoP) and mortal beings can only enter these unstable planes X number of times per week! (Hah)
Yeah, triggering the event is what I was talking about. However, I strongly disagree it wouldn't need to be limited. Remember, the loot scarcity and flatter item progression were keys of EQ, and stated keys of this game. If you allow for unlimited engagement of raid mobs, you also allow for an unlimited flow of loot into the game. And don't for a second think that "well, if it's a long grind then it won't be that much!"--I actually opened the gates on our server (Well, I was the leader of the guild that did.)--people will, and can, do ridiculous things to gain access to mobs.
The whole point of not having instances is, after all, really about creating competition through loot scarcity. If you have a way to circumvent the need for competition, you lose scarcity. Which means you need a loot progression curve more like WoW; because the main point of new content will be to replace old loot (Like it is in WoW). However, in a game with a flatter curve, and scarce items; the main point of a new raid target can actually be a higher availability of the current gear.
For example, if I have a high end raid, and the boss that drops chest plates spawns once a week--that means there are, at most, 48 chest pieces that drop
PER YEAR. That means the vast majority of the server will not get a chest piece. Which means I can make up another raid target, and also have him drop chest pieces--and he's still a valuable target, even though he doesn't have a loot upgrade. Why? Because the original source had such a scarcity. (Which, agrees with what you said about offering a lot of content to combat monopolization--but read below for eventual problems with simply creating a huge amount of targets).
The thing is though, that when you add instancing or summoning, your "supply" curve in a typical supply/demand profile is not only affected by time but also by population. Greatly decreasing time until demand is met. This is why controlling how much access each actor within the population has to loot, is so critical for limiting the bad effects of instancing or summoning mobs. (But even this has problems, as Kruegan mentioned earlier--guilds will create alt armies just to have more summons ect. But that can be combated with account wide limitations, I don't know, but ever system has problems).
However, I see the point that always being the empty-handed second in a race is a frustration on raid groups. One thing to keep in mind is that with a large world the number one raiders cant be everywhere. They take first choice or maybe are efficient enough for second choice too before you rally, but if there is enough content you should get something (bottleneck key mobs are a different topic, keep that shit solo/group only imo).
Yes, you can try to combat monopolization through adding more raid targets/conetent (And this is a good way to reduce scarcity, yes). But remember, we're still trying to maintain the balance between scarcity and availability. You don't want enough raid targets to make the scarcity go away (At least not in the current tier). Right? But if you don't have an absolutely MASSIVE availability, then monopolization becomes much easier--and that gives you accessibility problems like you see on p99 or the progression servers (People even resort to DDOS attacks, it gets so desperate.) I mean, after all--entire real market sectors fall to monopolization, despite millions of other sources of competition and whole governments trying to stop it. It won't be easy to prevent that in a game without breaking the market by making the resources (Loot) too common, heh. (I disagree a top tier guild might only get one or two--especially if raids are set up where all the mobs are in one place, like NTOV ect.)
In essence, if you have too much content, then gear flows in too quickly. There is no competition then--or it becomes what we would call a public goods dilemma in economics. It's hard to form a competitive market over it because access to it is so easy, and ubiquitous. But if you have it limited, monopolization becomes an issue (Which is what we discussed above). I think the best way to land in the middle of these two difficult problems, is to allow for small, discreet amounts of summoning/instances, to alleviate monopolization, while maintaining scarcity.
Edit: Sorry, lots of edits--trying to keep it under 5 pages. heh, economics is kind of my thing, so I love this aspect of competition in games.