Depending on how it is done, shortage of a resource does not need to cripple the realm, or cause undo hardship. It could be something that was nice to have, and that you'd want to have, but not a killer if you didn't. For example: If you have Metal in your realm/duchy, then recruits from the RCs in that realm/duchy could cost 2 gold less per 10 men. Nice, and you'd want it, but not a gamebreaker if you don't have it. You could probably do something with the others, too. Wood makes archer units a bit cheaper, and level 1 and 2 forts less expensive to build/repair. Stone makes level 3+ forts a bit cheaper to build/repair, and maybe a bonus on road maintenance. Etc. This would give traders a meaningful role, as well as put some strategic value on specific regions, but not overly penalize realms that chose to not do it, or couldn't obtain the resources.
The new trade system actually makes this a lot easier. You don't need to run around the whole island carrying stuff, and having to carry 8 different resources, etc.
