TRoS is essentially two things, a system and a setting. The setting serves well to illustrate some of the system concepts (rarity of the inhuman for example) but the system will work perfectly well with other settings.

So far the supplements I'm aware of are a bestiary, a book about combat, a book about magic and the fae and a book about thieves.

The bestiary I can see. What is the point of the others though? TRoS, to its credit, is a complete game in the core book. Like CoC or Pendragon. Now, obviously the TRoS crew want to produce supplements to support the game and fans like seeing new stuff. If the game is complete in the core book though it begs the question, what are the supplements for?

Setting supplements would be for players who were into the provided setting. Stuff on particular societies or adventures maybe. That I can see a point to, even though I don't personally like the setting. But that's not what's being produced as far as I can see. Rather what's being produced are rules supplements.

The trouble is, I'm not sure they're needed by and large. Thieves historically are just guys who steal, I'm not entirely clear what there actually is to write a book about. The main book makes fae rare, a whole book about them will I suspect inevitably lead to their being a whole lot less rare. The Flower of Battle may help some games, but the combat system is already both a bit on the complex side and tremendously robust. Does it really need expansion?

This sounds terribly negative, it's not. What I am saying is that supplements should have a point. Not be just for the sake of having supplements. TRoS like many good games is a complete game, you could play for years with just the core book. That is a great thing and a testament to the work that went into it. It does though mean that supplements need a bit more point than in many games, where a fighter splatbook adds value because the core book lacks useful detail.

Sorceror is an interesting example here. Sorceror is a complete game. It needs no supplements, particularly not mechanical ones. But, it has supplements and good ones. The trick there though is that each supplement genuinely adds something, showing how to use the game in a new setting or how to emphasise practically the kind of play Sorceror aims for. I think TRoS could take a leaf from that book.

So, what is TRoS about? What is special about it? For most people I think the Spiritual Attributes, not the existence of magic or thieves which exist in every other fantasy game on the market. So, a supplement about how better to use SAs, how to make them matter in play, how to use them to drive story, that I think would add value the way Sorceror's Soul adds value to Sorceror. A book though which adds on some new rules doesn't really, it doesn't bring anything new to the areas which are genuinely different.

What do others think? Are mechanical supplements really the best way to go or are there other types of supplements which might better play to the game's real strengths?

Well, for one.. supplements might be created in order to remedy things that were omitted in the core-book... things such as mass combat which is going to be in TFOB. Sorcery and the Fey most likely will have an expanded view on the magical system.
To me, there are alot of things that set TROS apart from such mundane RPG systems like D&D et al. First, as you stated, are SA's. These are how your character progresses, since there are no levels, hitpoints, etc.. and it also helps to aid in roleplaying. Secondly, the combat system is far more advanced than anything I've seen in D&D, being that there can be one-hit kills.. and simply because it is FAST. Now then, to the issue of the book on thieves.. from what I understand this is being done outside of Driftwood. And then there is the fact that Fey are indeed rare... and regardless of whether there is a supplement to them or not they probably will stay that way unless you as a GM go against what is said in the core-book... being that for the past 800 years, no Fey have been born. That tends to make something rare. Call it a 'limited production', like in exotic cars..such as Ferrari and Lamborghini... etc. Too many of these would lower the value of each individual one right? Exactly.
I take issue with you saying that TROS is a 'complete' game. If it were complete, I would not have to be researching stuff that is not in the book... I would not have to figure out how to do mass combat, and neither would anyone else. You say that a supplement for combat might be good for some games, I suspect it will be good for most games. Weyrth is not exactly the most peaceful place, there are wars, crusades.. etc etc. Not only that, but there are tiny little skirmishes in comparison. Such things as 20-30 people attacking the PC party. Now then, how in the heck are you going to go about resolving this situation in an efficient manner? Can you do it with the normal core-rules? I'd like to see that.
You say 'supplements don't add anything new to the areas which are genuinely different'. I say that supplements are just expansions of the previous set... they are amendments to a core-book which I assume was kept small for the sake of saving money and time. I eagerly await mass combat, combine that with sorcery and we're going to be having some serious wars going on.
With TFOB, it might not so much be mechanical as one might think. There might be expanded rules for the skills of true leadership, tactics, strategy, etc. There might be ways to influence the outcome of a skirmish through planning by the PC's, and therefore there might be some suggestions and such in TFOB for those people who IRL have no knowledge whatsoever on tactics and the like. Take a poll, and find out that not everyone in the universe knows what a flanking move is, or how to best use cavalry in a situation, etc etc.
You may be viewing supplements as a bad thing, fearing that this will turn into D&D with 20 books or something. It wont, first off... there wouldnt be that much shit that would need supplementing.. secondly, that would be waaaay too damned expensive over the long run, and thirdly.. if there isn't enough interest when the thing gets off the press.. Jake loses money. And Jake is relatively smart :-D, and as a businessman I think he wouldn't want to take risks like that

So there's my view on this.
-Ingenious

I think that the proposed supplements so far are apporpriate. You are right in that TROS is complete enough that a supplement isn't needed...but that is really the whole idea of a supplement. It isn't needed by everyone but addresses issues important to some. For example, I can't wait for good mass combat rules. Others might not care. I'd also like rules for fortifications.

In general, I prefer supplements that provide rules and mechanics, not just pseudo-historical information. That is easy enough to look up in the library for free.

I wouln't mind some historical supplements that would provide some background for different time periods. eg dark ages, classical period, early medieval...However, I read a lot of history and know enough about the periods (at least in terms of military things) that I don't need these supplements.

NT

Ron has pretty special criteria for a supplement. Here's mine:

1-The fans want it.
2-I would buy it and use it.
3-There something that I want in the game but left out of the core rules, intentionally or otherwise.
4-Having read it, the supplement is useful and can enhance, change, or otherwise improve play for some groups, though not all.

Bam.

Jake

I just find it funny that the bestiary is the one supp that balbinus thought was needed when I myself saw it as a really fun and neat extra but not necessary
per se. anyways, I am really looking forward to the flower of battle especially given the teasers here and when sorcery and the fey comes around teh bend I may look into it as well despite never running games with magic and subscribing to the "if you see a mage in my campaign and you have no magical protection and value your life, run like a sissy!" philosophy.

Meanwhile, the Flower of Battle seems to me to be the most unnecessary (which probably won't stop me from picking it up), but I'll be snapping up Sorcery & Fey in a heartbeat. heh Just goes to show you, it takes all kinds.