Wish allows casting a lower-level spell 'without meeting its requirements'.
A major restriction to many spells is that it can only be cast on a willing creature. Can I use wish to avoid this requirement and enjoy the cheese that follows?
Maybe I'll throw the Big Bad into permanent suspended animation via sequester, then toss his body into Mt Doom, for instance.
By RAW, do creatures have to be willing for a lower-level spell duplicated by Wish to work?
As long as you can designate a target for a spell (and satisfy all the other requirements to cast it, of course), you can cast that spell. The section on spell targets starts (emphasis added):
A typical spell requires you to pick one or more targets to be affected by the spell's magic. A spell's description tells you whether the spell targets creatures, objects, or a point of origin for an area of effect (described below).
You are required to pick targets for the spell, but the targets you pick may or may not be valid. They might not even be the right "kind" of target. To take the example from XGtE's1 section on invalid spell targets, the Charm Person spell targets "a humanoid you can see within range". You might be fooled into casting this spell at any number of non-humanoid targets. The section gives the example of a vampire, who appears humanoid but is actually undead. However, you could even end up targeting things that are not creatures, such as an illusion created by Major Image, or a stone carved and painted to look like a humanoid. As for what happens when you designate an invalid target:
If you cast a spell on someone or something that can’t be affected by the spell, nothing happens to that target, but if you used a spell slot to cast the spell, the slot is still expended.
In fact, you don't necessarily need any target at all to cast a spell. Any spell that can be readied can be cast without choosing a target, since the whole point of casting the spell in this way is to ready it to be released later (typically after an appropriate target has presented itself). For example, if you ready a Blight spell with a trigger of "when I see an enemy", you are casting it with no specific target in mind.
So, we can bring this back around to your example of casting Wish to duplicate Sequester, targeting the BBEG, who we can presume would rather not be sequestered. Wish will duplicate the effect of Sequester, and Sequester only has an effect when cast on a willing creature. Hence, the duplicated spell has no effect because the target is invalid.
(As always, because you're casting a spell that can potentially do anything, you can wish for the target to be affected even though it is invalid. But this would no longer be duplicating the effect of a spell, so it comes with all the associated downsides of using Wish in this way.)
1It's worth nothing that the rules in XGtE are considered optional, but in this case I would argue that they are mainly just taking the implications of the core rules and making them explicit (do you really need to be told that a spell can't affect an invalid target?), so it makes sense to use them as a default.
The standard use of wish states (PHB, 288):
The basic use of this spell is to duplicate any other spell of 8th level or lower.
Therefore by using wish to duplicate the casting of a spell of 8th level or lower, you will avoid stress.
However, if you change the spells, then you simply aren't casting that spell. You are no longer duplicating that spell, and instead you are now casting something different and special that isn't that original spell.
In this case, you're not ignoring a requirement, you're changing the spell text from willing creature to unwilling (or some other variation of change).
This means that there is stress. If you want to avoid the stress, just avoid going outside the normal wish guidelines.
The wish spell lets you ignore "requirements" but it never defines what sorts of things are "requirements".
The top-voted answer at What exactly is ignored in the "requirements" of a spell when Wishing for it? acknowledges this, but then attempts to derive a rules definition of "requirements" based on the standard English definition. That's not Rules As Written and we shouldn't claim that it is.
This spell is ambiguous, and when a rule is ambiguous you have to ask your DM to resolve it.
As you've pointed out, the exploit involving sequester is completely broken and a DM should not allow it in their game.
