By JJ: To assist Hugo nominators, listed below are the series believed to be eligible as of this writing for the 2020 Best Series Hugo next year *†.
Each series name is followed by the main author name(s) and the 2019-published work(s).
Feel free to add missing series and the name of the 2019-eligible work in the comments, and I will get them included in the main post.
I just ask that suggesters (1) first do a Find on author surname on this page, to check whether the series is already on the list, and (2) then make an effort to verify that a series does indeed have 3 volumes, that it has a 2019-published work, and that it has likely met the 240,000 word threshold; in the past I have spent a considerable amount of time trying to verify suggested series, only to discover that they had fewer than 3 volumes, or nothing published in the current year, or weren’t anything close to 240,000 words (e.g., children’s books). Self-published works may or may not be added to the list at my discretion.
Note that the 2017 Hugo Administrator ruled that nominations for a series and one of its subseries will not be combined. Therefore, when nominating a subseries work, think carefully under which series name it should be nominated. If the subseries does not yet meet the 3-volume, 240,000 word count threshold, then the main series name should be nominated. If the subseries does meet that threshold, then the subseries name should probably be nominated. This will ensure that another subseries in the same universe, or the main series itself, would still be eligible next year if this subseries is a finalist this year.
The 2018 and 2019 Hugo Administrators ruled that the 2017 Best Series Finalists, although the result of a one-time category, were subject to the same re-qualification requirements as the 2018 Best Series Finalists, and it is likely this will be the rule going forward; bear that in mind when making your nominations.
* ineligible series are preceded by an asterisk