This is a compilation of all the romance based isekai I know of.
Isekai is a specific category in the fantasy genre that is created on the basis of worlds/universes that are not (modern) Earth (aka otherworlds). The most common otherworld settings presented are medieval/historical, dungeons, simulations/virtual games, novels, martial arts, and magical fantasy. Isekai is also heavily based on the following four concepts: transmigration, reincarnation, regression (time travel), and summoning.