In OTL, there was a broad principle that the Holy Roman Empire could not contain further kings, since (if I understand right) there was only a King of the Romans who became Holy Roman Emperor. Anyone who was a liege (in theory) of the Emperor could not claim the title of king, since that implied equality with the Emperor.
An exception was made for the title of King of Bohemia, since that had been a pre-existing title for territory which was gradually incorporated into the HRE. Most famously, the Electors of Brandenburg became Kings in Prussia (and eventually Kings of Prussia) while remaining within the HRE as Electors of Brandenburg.
What other HRE nobles could plausibly become kings while remaining part of the HRE, so that all of their HRE and non-HRE territory would be considered de facto part of their kingdom (a la Prussia-Brandenburg), or their outside territory is integrated into the HRE? This does not count for something like Denmark, where the Kings of Denmark were also Dukes of Holstein, since they were pre-existing kings outside of the HRE, and there was no question of Denmark becoming considered part of the HRE. Nor would examples like the Electors of Saxony becoming King of Poland, unless there's a way to have all of Poland considered part of the HRE.
Are there any plausible ways that other kingly titles could be created/re-created? Perhaps, say, something involving the title of King of Burgundy being resurrected for some portion of holdings outside of the (later) reduced HRE borders, so that there's now a King in Burgundy like there was a King in Prussia in OTL?