This would be a huge change from a 4e standpoint, but an interesting one. Right now you've got experience points given out in measured-by-encounter batches for defeating foes and treasure given out in measured-by-level parcels. Messing with one doesn't change the other e.g. if you decide there's going to be no treasure here then so be it, the party still get x.p. for what they beat. But if you bundle x.p. in with the treasure parcels that means a DM would be somewhat obligated to give out treasure no matter what but also means adventures can be designed without such a combat-first background. Hmmm....
How do you handle PCs creating their own items to avoid double-dipping i.e. getting a bunch of x.p. for claiming lots of gold (as opposed to items) then turning around and plowing that gold into item creation, thus ending up with x.p. *and* items?
The simple answer - one I like but not many others seem to - is to simply take item creation out of the PCs' hands. If you want to make an item then fine, but you'll be home for a year or two while your companions keep adventuring...
The XP change is a huge difference from 4E, but remember this is spirit of RC, not spirit of 4E or a blend of the two. 4E is just being mined for mechanics and design. Some of those mechanics will necessarily pull a bit of the 4E spirit into the game, but it has to fit that RC spirit to be included. Some people might disagree, but I don't think it is counter to the RC spirits to ask, say, "How would RC handle skills in a way that fits the rest of the game, if the authors had access to modern game design?" Me, I'd do it something like 4E skills, but that's just one persons' opinion.
As for the treasure, gold and magic, issue, maybe I should explain where I'm coming from with a more direct answer to Dausuul's original question, from the lower level perspective:
I see the spirit of RC as, broadly, that relatively weak adventurers go into a dungeon or the wilderness or a rough town. Through their cunning and power, they defeat the opposition, albeit with a real risk of failure. They get some treasure, and get more power.
In contrast, I see the actual mechanics of RC, as written, as a group of very weak adventurers go into hideously stacked against them environment and perish. Every now and then, they get lucky
and clever enough to get some treasure and get more power. Lucky
or clever seldom is enough.
4E solves this discrepancy by making the low level guys not so hideously week (relative to the opposition), and by a strong focus on balance.
RC DMs solved this discrepancy in a variety of ways. I'll list a few I used (I played it a lot as Red and Blue box):
1. Heck with the spirit--just play it in the spirit that the mechanics produce. No more discrepancy.
2. Fudge to preserve the spirit.
3. Start higher level, avoid the problem.
4. Make mechanical changes around the edges (max hit points, better stats, and so forth in increasing elaboration).
You might note that none of those are truly acceptable in a modern design. If the spirit of the game--as conveyed in the tone of the characters, background material, etc. is as I've said it is, then characters played by RAW ought to be a bit more survivable. OTOH, spirit is nebulous. So my suggestion is that a modern design should provide a default means for RAW play to get something closer to Fafhrd and the Gray Mouser, but it should
not prohibit something like the original RC played via RAW. I guess I'm saying that the spirit of RC is something of a moving target.
Making power mechanically tied to gold and other non-magical treasure is a way of squaring that circle. (There might be other, better ways to do the same thing.) Leaving the magic items out of the equation prevents the 4E problem of expected equipment, while still allowing the rest of the treasure to drive personal power. You could use the 4E inherent bonus solution, but then that gets into the scaling issue that pawsplay raised. (I agree with him that the scaling should be more constrained.)
I hope that made some kind of sense.
Edit: Realized I didn't finish answering Lanefan's point about magic item creation. I'm envisioning some fairly strict restrictions on item creation, with gold not being the main bottleneck. There would be costs, mainly for flavor and secondary bottlenecks to a flood, but the main resource should be time. For example, to keep it simple, I'd be perfectly fine with something like allowing spellcasters to make any spell into a consumable item, taking one day per spell level to perform, and then requiring 1 week per spell level of "rest" before they can do it again. (Not actual "rest"--just a recharge time on when they can do it again.) Though probably I'd bring in 4E rituals, twist them a bit to fit RC better, and put a similar recharge restriction on them.