Ore dake haireru kakushi dungeon

 Another Fantasy Harem series that gained recent popularity with the anime adaptation and if To Love Ru took place in fantasy setting, Ore dake haireru kakushi dungeon comes closest to it. For a fantasy series, it is not particularly exceptional but it excels at the harem part with the generous amount of fanservice it come with. In terms of story formula, i think Hidden Dungeon shares close similarity with Danmachi as the main protagonist is a fledgling adventurer who tried to make his way to the top of society with the help of powerful patron and inevitably getting himself a harem of cute girls flocking to him. As for the Hidden Dungeon Main Protagonist, he get access to a hidden dungeon where he is able to quickly level up with his overpowered set of skills given by a mysterious beautiful woman imprisoned in the dungeon. The powerful skillsets granted to the main protagonist enable him to gain powerful skills which comes at a price by spending skill points gained by seeking pleasure and for the Main Protagonist case, he needs to do lewd things with his harem which is the main source of Fanservice,

For the Fantasy part, Hidden Dungeon is subpar due to typical munchkin stuff. However the harem aspect become the series strongest selling point as it is capable to deliver maximum amount of fanservice without crossing the H Border which makes it one step behind Peter Grill no Kenja which is another great manga for this niche. If you need some fix for Fantasy themed harem, Ore dake haireru kakushi dungeon worth a visit but if what you wish is for some straight action, perhaps Isekai Harem Monogatari can be considered.