Mostly just posting for my own benefit. These are the things I'm thinking about these days. Mostly things that have come up during play.
Alternative casting rules
I'm finding that using a d6 dice pool is resulting in spells either being more or less automatic (for easy spells) or more or less impossible (for hard spells). I had thought there would be a tension in rolling each round to cast spells, but it turns out that currently most spellcasting is outside of combat.
And spamming of low-level spells (especially detect magic type things) annoys me. It makes magic very mundane.
So, a new system is required for that.
I'm finding that magicians, especially, are not enjoying combat as much as perhaps they could, as they currently don't really have any combat-applicable spells.
So, I'm thinking about what spells I can add that keep the flavour I want for magicians, but that can be used in combat. Illusions are something I'm leaning towards.
Descriptive damage rules for large creatures
The descriptive damage system I outlined a while back (in this post) is working swimmingly for humans and humanoids.
It does not currently have anything to say about large creatures. The couple times I've had to run large creatures since implementing descriptive damage, I simply used a basic hit point system. And that worked ok, but I'd like to be able to include the grit and interest of the descriptive damage system into fights with giant creatures.
Making Burglars more interesting
Both in combat and out. The main thought I'm having at the moment regarding this is making them a little better at combat than they currently are, and including some social abilities, persuasion-type stuff. We shall see how that plays out.
XP and Advancement
For want of a better system, we're currently just using a negotiated level up system. When the party has done a remarkable feat (or a few), we chat about it, and decide if they're deserving of a level up. This is working fine, and is essentially what most systems boil down to in any case, but perhaps a little more guidance would be good for a published game.
Something sort of similar to Paolo's system in Adventure Fantasy Game sounds plausible, but instead of treasure being the driving force, using great deeds (which he also supports, actually). His advancement system is worth checking out, as is his game.
While I've been a defender of XP for Gold, and while that is a perfectly workable system, I'm not sure it's right for Spells and Steel. For one thing, cash is scarce in my world, and I don't have the kind of economic system in place yet that Alexis does, so I'd have to be coming up with cash equivalents for all the treasure...
So what I'm thinking at the moment is guidelines for the kinds of feats and challenges are appropriate to trigger a level up. More thought is required.
Things that are working well
Descriptive damage is great. I've also got some rules on hand for infections and disease, which I'll get around to putting up when I feel like it.
Combat in general, and the Fighter class are working very well. Which makes sense, since that's where a lot of my effort has gone to date.
Classes are strongly differentiated, which is something I wanted, but I'm starting to wonder if that's such a good thing.
I've been avoiding "+X" magic weapons, and am loving it (something I talked about before, as have others). The players actually have yet to identify any of their magic items, but they are using them and benefiting nonetheless.