Absolutely create your own UI Design Templates to meet the User Experience needs of your client if the out-of-the-box ones will not do. This can be accomplished by creating a new Section rule in Dev Studio and selecting "Design template" from its Settings tab, like the One Column OTB template below:

  Avoid one mega-Template that attempts to cater for all possible layout options. Also avoid creating one template per Section. The right number of templates is somewhere in between, informed by a User Experience consultant who has engaged with your Client to form a vision.

If a User Experience specialist cannot work with the delivery team from the very start, you can help them when they engage later by collaborating with your Client and understanding - broadly - the different Layouts needed and how many Regions comprise each Layout.

With this information your team can build "draft", un-styled Design Templates that are sufficient to then associate to the Views of Process Steps in the earlier Sprints. Associating the data needed for each View to the Regions in the "draft" template such that a working process can be completed subject to finalization of the templates.

When the User Experience specialist eventually engages and finalizes the vision, the additional work to these Templates may be limited to styling and re-laying out the existing draft Templates if the number of Regions for each was correct. Changes made to Template T in Sprint N take effect on Views configured in Sprint 1 attached to Template T ... without having to refactor those Sprint 1 Views.

Discussion on this topic was sought from the LSA Data Excellence (Pega 8.4) webinar conducted in July 2020. The webinar and its full set of discussions that arose from it are available at LSA Data Excellence: Webinar, Questions & Answers.