A brand identity is expensive to change and a campaign layout is not. That difference should decide how much of a current design trend you let into each. The test I apply is simple: if a choice will look dated before the asset is due for replacement, it belongs in the campaign and not in the identity.
Choices that hold up
Generous space is the most reliable of them. It has read as considered and expensive for decades, it costs nothing, and it makes everything placed within it look more deliberate. When a layout feels cheap, spacing is usually the reason.
A restrained type system holds up as well. Two families, four sizes, and a consistent scale will carry an entire brand and will still look coherent when applied by somebody who was not there when the guidelines were written. That last part matters more than most identities allow for.
Real photography of real people and real work outlasts illustration styles by a wide margin. Stock illustration in particular carries a very legible date stamp, because everybody adopts each new style within the same eighteen months.
Contrast that meets accessibility thresholds is worth treating as a rule rather than a preference. It ages well, it is legally safer, and it means your text stays readable on a phone in daylight, which is where a great deal of it will be read.
Choices that date
Whatever gradient treatment is currently everywhere will look like the year it came from. That is fine on a campaign page with a six-month life and a problem on a logo with a ten-year one.
Heavy motion is similarly time-stamped, and it carries a cost beyond fashion. Animation that delays content is a performance issue on the devices most of your audience is using, and it is the first thing to fail when the connection is poor.
Novelty typefaces are the fastest to age of all, because the ones that get noticed get adopted, and once they are adopted they read as a period marker rather than as a voice.
Where each belongs
Identity assets, the logo, the core palette, the type system, and the layout grid, should be built from choices that hold up. Campaign assets, social creative, landing pages, and event material, are where a current treatment earns its place, because they are cheap to replace and their job is to feel like now.
Keeping that boundary clear is most of the discipline. It lets a brand look current without repainting itself every two years.
The question that settles most arguments
Will this still look right when the asset is due for replacement. For a logo that is ten years and the answer is usually no, so leave the trend out. For a campaign page it is one season and the answer is usually yes, so use it and enjoy it.
Most design disagreements I have sat through were really disagreements about that timeframe, and naming it out loud ends them faster than any mood board.