Now, the problem with Glorantha is not that the setting is, in and of itself, not an interesting, exotic affair that has a dedicated core of fans....it does. Glorantha is, indeed an exotic, unusual setting. It's probably close to Barker's Tekumel in many people's eyes, or at least settings like Jorune or Talislanta. Worlds that are sufficiently exotic that the rules of play are bent around the frame of the setting to support it (rather than, say, a set of rules on which one drapes the setting). Because of this, Glorantha has a reputation: it's the setting you choose when you want something really weird and different, something which is not in the least bit conventional. There's absolutely nothing wrong with this, of course....other than the fact that Runequest has been presented as a more universal system for 31 years now since Runequest 3 was released in 1984ish, where the default setting was actually Mythic Europe and Glorantha got a supplemental book that added the setting to the rules.
This was hardly even a problem back in the day because Runequest 2 hardly included enough information to do more than get a vague sense of what the world of Glorantha was like. I ran scenarios ostensibly using the map, but my attention do detail ultimately led to adventures set in some corner of my own world (look for the location of Blackwell in the World of Lingusia). But of course that was 1982....and in the intervening years game design has changed, along with the "minimum expectations" of gamers and their settings.* As such, we all sort of anticipate that the next Runequest will have the following criteria:
1. It will be a setting with a system wrapped around it (maybe)
2. It will be heavy with Glorantha-focused content; so instead of a chapter on world building we'll get a Glorantha-focused chapter on the setting. Instead of charts and tables for general fantasy medieval adventuring we'll get charts and tables for Glorantha. Instead of all-purpose cultural details (which really are an artifact or RQ3 and later) we might get cultural details on Glorantha locales.
3. RQ4 will maybe....at best...focus on a specific era of Glorantha, which as I understand it has several ages (such as the second age of the setting that Mongoose's RQ focused on).
So....what that means is that it will be one of those games. Like Earthdawn, Empire of the Petal Throne, Talislanta, and others, in which what you are buying in to is a complicated setting, not necessarily a toolkit rulebook. Problem is, toolkit rulebooks are all the rage these days; the idea of a core system of rules which then supports multiple settings is very much a part of contemporary gaming culture. Runequest will be designing against this principle. That could work out well for them, sure....but it might backfire, too. Especially because Glorantha has a reputation among many gamers as being a difficult setting to penetrate. Few "setting-dedicated" rulebooks do well.
I like to define Glorantha as an "opaque" setting...very difficult to penetrate. Glorantha has an appeal to gamers who feel that something is not worth their time unless it has at least two of the following traits: it is obscure, it is academic and/or it is difficult to master/understand. Glorantha fits all three criteria.
Runequest 4 could avoid this problem by providing the skeleton of the Glorantha setting and still retaining "toolkit" elements. 13th Age does this....it has a world setting implied in the rules with some robust chapters providing necessary details, but the rules also assume you can and will do your own thing with it. As such 13th Age walks a fine line between predetermined world and open toolkit in it's design. RQ4 could do the same.
If RQ4 remains a dedicated book for Glorantha then there's a lot of content it won't need: Glorantha has defined cults and gods, for example; so we wouldn't necessarily get rules on how to make our own. Gone also will be other rules we've come to appreciate, such as designing orders and guilds, magic systems which may or may not be supported in this revamp of Glorantha, and specific magic systems retooled to reflect the Glorantha-specific flavor, making them hard to extract for use with non-Glorantha settings.
And of course the book will be big, and thick, because Glorantha is a monstrously large setting defined in countless obscure tomes. Dedicates to the setting love it...but you have to really, really love it to hold to such a commitment. Sure, RQ4 could garner lots of new fans, but the problem is that there are an enormous number of RQ fans out there who have been enjoying the system without a core setting for three decades now; and a lot of us specifically liked how Glorantha was carefully excised from the core rules for four of the six editions of the game.
This all ultimately points out that the return to Glorantha, while fine, is the real problem here; we're all about to get a combined rulebook and setting that no half of RQ's fanbase don't really want, haven't really wanted since about 1984. And of the other half, the Glorantha fans who do want a return, I am unsure of how many are going to really want to return to Runequest as a system when they also have HeroQuest, which has struck me for years now as a great example of the system designed to support the setting. Is RQ really that suited to Glorantha?
There has always been something of a split on this, though. I suppose we will now get a chance to see if a re-Gloranthified Runequest based on RQ2 will take off and be a success.....and I guess fans of RQ the toolkit system (but not Glorantha) can see where the Design Mechanism goes.
EDIT: worth mentioning that in the end, Runequest was
My own preference....well, we have a lot of choices out there, and in the end games don't go away until people stop playing them. I'll continue to reserve my feelings on the matter until we see what 2016 brings us.
*Look at how many people complain that the Sword Coast Adventurer's Guide is too skimpy to run Forgotten Realms scenarios. I personally think it's loaded with useful GM stuff, plenty of campaign material.