I always like the week between Christmas and New Years to focus on cleaning up our instance from the past year so we start fresh. These are reviewed as needed throughout the year but this is the 'Big List'. My list is building but here's a snapshot of what I mean.
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I think many of ours ended up on the list in the system maintenance user group https://one.workfront.com/s/question/0D54X00006msfHdSAI/user-group-followup-july-27-2021-workfront-s... -- and anything that didn't make this list made it over to Doug Den Hoed's health checker. But I can summarize my goal as being able to look at everything that is considered "an object" and say something about its maintenance.
e.g. Workfront considers Portfolios an object, so what can I say about this from a maintenance perspective?
... and so on and so forth: programs, templates, projects, tasks, issues, documents, document or proof approvals, approval processes, milestone paths, reports, dashboards, users, groups, teams, roles, layout templates -- the list goes on -- what is it you can say about each of these objects from a maintenance perspective?
The great thing about Doug's list is it's already pretty organized in order of "most bang for buck"... but that's just my opinion.