If you're craving that exact blend of time-slip romance, Scottish atmosphere, and wide, generational scope that 'Outlander' delivers, my top recommendation is Susanna Kearsley’s novels—start with 'The Winter Sea'.
Kearsley writes the kind of haunting, slow-burn time-slip that feels like a foggy walk along a coastline at dawn: present-day protagonists who become entangled with past lives and old secrets. The prose is quieter than Diana Gabaldon’s, but the emotional payoffs are equally satisfying. After that, her other books like 'The Shadowy Horses' and 'Mariana' scratch the same itch in slightly different historical settings.
If you want something broader and more epic, read Deborah Harkness’s 'All Souls' trilogy beginning with 'A Discovery of Witches'—it swaps Highlands time travel for witches, vampires, and deep archival research, but it has the same sweep and romantic intensity. For historical romance with war-era stakes and gut-punch emotion, Paullina Simons’s 'The Bronze Horseman' trilogy is a tidal wave of feeling. Personally, I bounced between Kearsley for the mood and Harkness for the plot complexity, and both kept me turning pages late into the night.