Practically every adventurer is going to come across at least a +1 weapon at some point, unless the campaign is extremely low magic.
I really hope that next edition wants to get finally get rid of this kind of assumptions.
This is exactly the kind of assumptions that made 3e so rigid against playstyles that differ from the edition's chosen one.
If 5e wants to be inclusive, they really should kill this type of assumptions, all of them or at least as many as possible. It doesn't matter whether the assumption of +1 weapons is much milder than 3e assumption of +N every M levels, they have the same consequence.
We should already have had some ideas on how many people do
not want to have even +1 weapons in their game, just by reading these forums everyday.
They talk about using creative means to defeat such creatures, but that isn't how it goes. What happens is that one or more of the players has to sit there bored and useless while the rest of the party kills it. If a creature is immune to weapons, the spellcasters kill it. If the creature is immune to magic, the weapon-users kill it.
This is how it goes exactly when players don't want to be creative, which is a totally legitimate playstyle itself because creativity requires some thinking effort. The beer & pretzel gamestyle is supposed to be relaxing, and many groups want to just bash things without having to think too much, and that's not badwrongfun.
But then there are others who prefer a more engaging gamestyle. That's what you get exactly when your most obvious tools (the fighter's weapon, the wizard's spells) stop working as usual, not necessarily stop working at all of course.
The game needs to provide unexpected challenges to be engaging for this gamestyle, and immunities are one of the many possibility. There are anyway plenty of other monsters which are instead meeting the expectations and just need to be bashed to death. A good DM learns to use the monsters more appropriate for her group's favourite playstyle.
Instead, this should be an optional suggestion for customizing monsters.
Would work for me. I like the trend set by the playtest's bestiary of having a sidebar for optional additions to many creatures. If this works well enough for you when you want to ignore those troublesome features, then it works well enough for me when I want to use them!
I actually think this could be the best approach for any controversial monster special ability such as
energy drain.
If we can get an indication of XP adjustment when using the optional special ability, then great, but even if we don't I wouldn't mind that much.