Recommendations

not very much similar, but both dramas have very healthy positive and funny vibe (though Fantastic touches very negative subject and has sad parts it still manages to be positive drama), realistic relationships and characters, multi characters and still quite 3d all of them, without unrealistic villains (though Fantastic has very pad people as one family but still realistic ones), primitive cliches, healing...
there are no annoying love triangles, I mean it is in both drama but very healthy when SML and FL are reall good friends without broken hearts.

both are really good dramas, with one look not some new or extraordinary plot but really good watch, dramas that enjoy you (and I's saying it when 80% dramas are boring for me after 5-6 episode)

Both are about Huo Yuanjia a legendary chinese martial artist that made a big impact in China. Both have similar fight scenes like the wrestling/boxing match and vs the Japanese. Both have famous martial artists starring as Huo Yuanjia. Fearless has Jet Li while Heroes has Vincent Zhao.

This story is about Shi Yi, a gentle pleasant and low-key but industry's top voice actress, who ran into the elegant returnee chemistry professor Zhou Sheng Chen at the airport one day. They gradually develop a mutual understanding in getting along with each other, and join hands to preserve traditional craftsmanship and overcome many storms as they decide to accompany each other for the rest of their lives...

both dramas show entertainment industry and real lives / relationships behind dramas;

both dramas show quite realistic relationships / romances (TWTTLI is more realistic, Fantastic in more entertaining and good watch), without endless misunderstandings and primitive cliches.

both dramas have high class actors.

If you have watched one of them, highly recommend the other. Love element, great acting and monsters in both

Set Play and Call Boy are both films about young male prostitutes that sleep with older women. In Set Play de ML is a minor, in Call Boy the ML is an university student. Both show explicit sex scenes.

Don't know why but i thought about this drama while watching Bulgasal.Both have monsters and strong female leads.

both has strong female lead
both has romantic affection
unexpected twists
exciting to watch, not a single second was boring so far with Bulgasal (currently airing) and till the end with Vincenzo

Similar aspects /elements between these two dramas. Bulgasal so far has less gore and has a fantasy element to it but both a have romantic relationship and I think people who liked Happiness might enjoy this one too!

Not necessarily related but lovers of one show might enjoy both.
Both have that sci fi element mixed with fantasy and monsters.

Both these dramas are centered on a mythical
and immortal creature who used to be human and whose goal is to find the person who'll be able to help them end the curse they are subjected to.

The dramas are high-budgeted with good acting and high production value and cinematography.

The of this drama is Cheng Xiao. About Romance. Although this is about xianxia or wuxia, the female main lead of the two drama is the same, so watch this drama if you like Cheng Xiao. This drama is also had a good storyline and will not make you bored while watching.

Both are dramas with female leads which are cute, fragile ladies with glasses on a noble government position which are trying to straighten up a social outcast by trying to solve some of their problems. The main difference is that in Gokusen the heroine uses martial arts to scare of the people, which are trying to hurt the people she is trying to protect... In Zenkamono the heroine is left alone with the system.

Both movies are sport related.
Both movie showing sport in countryside, out of city environment
Showing pure, simple, joy in sport.
Team work.

Both are a female leads slice of dramas which operates with the daily work and life condition of a person. Danda Rin is about work conditions and Zenkamono is about people on probation.
Both are very inspiring dramas that shed lights on the importance of those people's jobs.