A massive improvement to Marketo's field management capabilities would be create your own folders for better field management.
HubSpot, a competitor, already does this.
Being able to create your own folders for managing fields would improve admin's ability to keep track of what fields already exist as well as their use case.
A good example of this would be for email compliance related fields, or a preference center, where it's important to keep track of all the fields you use, and why you use them.
A similar ideas was published a few years ago - https://nation.marketo.com/t5/ideas/ability-to-group-custom-marketo-fields-in-various-folders/idc-p/260407
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