Marsha Canham’s Highlands series, The Pride of Lions, The Blood of Roses, and Midnight Honor.  There’s no time travel, but there’s plenty of adventure and romance set during the same Jacobite Uprising.

Sara Donati’s six part Wilderness series: Into the Wilderness begins this multi-generational romantic family saga set during early American westward expansion, complete with a a small Outlander tie-in.

If well-researched epic historical saga is your thing, try anything by Phillipa Gregory. The White Queen series, set during England’s Wars of the Roses was also adapted into a Starz series.

For more in the vein of war-torn couple who can’t seem to keep it together or in the same country, try The Bronze Horseman series by Paulina Simons.

For a time traveling Knight in Shining Armor, try Jude Deveraux.

And, last but not least, there always Gabaldon’s Lord John Grey spinoff, and several Outlander short fiction options.
……Is it too soon to start talking about book 9?