Taiyou no Ko Esteban

Both of these anime take place in the past and feature stories about rediscovering ancient technology that is beyond the wildest imagination and power of the modern civilization. Both also have relations to ancient Atlantis. 

Native main characters, soft fantasy, adventure/rs. Same kind of fantasy-slice of life good feel 

Liked the dreamy references to native South-American culture in the Mysterious Cities of Gold ? Flying Luna Clipper is nothing but that ! Also features another yellow flying bird-plane thing ! 

Both series are about a group of friends (2 x male, 1 x female) finding together one after the other and embarking on a journey. During their travels, they explore dungeons where they find mysterious artifacts. The protagonists also cross various human communities of the world they live in, and are confronted with their good and bad sides. Furthermore, the protagonists deal with characters who are not exactly good or evil and whose actions are not always predictable. This makes it pretty interesting to watch the path that the protagonists will take in my opinion. If you want to join treasure hunting protagonists in a world full of intrigue, conflict and wonders, both series may be for you. read more 

Both anime are about adventure, and share similar themes of a lost treasure. Taiyou no Ko Esteban is about exploring the Americas, and looking for the lost city of Gold.  

It's not so much about the similarity of story line or characters as it is a commonality of look and feel. I really think if you like Romeo no Mysterious Cities of Gold you will enjoy the time you spend on the movie The Boy Who Saw the Wind. 

Both are classic adventure anime from the 80s which both were produced/written/directed (depending on the title) by Jean Chalopin and Bernard Deyriès together with Japanese Studios. The also both have soundtracks created by Shuki Levy and Haim Saban, and both (English) dubs are made with the same voice actors. 

Made in Abyss and The Mysterious Cities of Gold are adventures series involving children who embark on a journey to find answers and reach a hidden location. Despite their age, the main characters are resourceful, tenacious, and courageous. Each character contributing their skills and abilities during the journey. There are obvious differences between Made in Abyss and The Mysterious Cities of Gold, most obviously their character design, animation style, and settings (fantasy world vs historical fiction). However, if you can accept the timeless qualities of The Mysterious Cities of Gold then you will discover a hidden gem.  

An adventure series is best illustrated with a quest where plot progression requires travel. Each episode advances the plotline as characters discover truths and reach their destinations. Many anime series are classified as adventure despite not delivering on this basic definition. The Mysterious Cities of Gold and Fena: Pirate Princess are pure adventure series. With its longer episode count, the Mysterious Cities of Gold is a better anime with its better characters, comedy and plot development. On the other hand, Fena: Pirate Princess features action scenes, and comedic romance. Your overall enjoyment of both series will also depend on your reactions to their final mysteries being resolved.  read more 

The child protagonists in these series are about the same age when they embark on a ship to find a long lost treasure. They both form a close relationship with an experienced sailor. If you enjoyed the mentor-protégé connection between Esteban and Mendoza in Taiyou no Ko Esteban, then you will most likely feel the same about Jim and Silver's in Takarajima, and vice versa. Both Mendoza and Silver are charismatic yet cunning leaders and father figures to the young protagonists while pursuing their own agenda. They will do anything it takes to find the gold. Both series are old-school children's anime that have adult characters in prominent roles. read more 

Are you looking for a very good adventure? Castle in the Sky and The Mysterious Cities of Gold feature children who are pursued by government agents as they seek to discover the remnants from a lost civilization. Despite their age, the main characters are resourceful, tenacious, and courageous. The Mysterious Cities of Gold was my favourite childhood anime. Castle in the Sky is a childhood movie for many others. Nothing beats a good nostalgic title that retains its quality when you're older.