Well, without reading the actual 'birdy the mighty naked fanfic', it's hard to say precisely. It could be a fan - created story that delves into the deeper emotions and experiences of the characters in the 'Birdy the Mighty' series. Maybe it looks at how the characters react when stripped of their usual defenses or status, in a more symbolic sense of the word 'naked', and then takes them on a journey that is unique to this fan - made narrative.
You could try looking on specialized anime - related fanfic communities or forums. Sometimes, fans gather in these places to share and discuss their fan - made stories. However, the title seems a bit unusual, so it may not be as common as other 'Birdy the Mighty' fanfics. Also, make sure to respect the terms and conditions of these communities when searching for it.
One idea could be to crossover Birdy with a popular mecha anime. Picture Birdy in a Gundam-like universe, fighting side by side with giant robots. Another could be a crossover with a magical girl anime, where Birdy has to learn some new magical powers to save the day. Or perhaps a crossover with a detective anime, and Birdy uses her skills to solve complex mysteries in a new setting.
Obviously, Birdy herself is a very popular character to focus on. Her dual nature as an alien and her life on Earth make for a lot of interesting storylines. Tsutomu is also a key character. His relationship with Birdy, from being initially shocked to gradually becoming her partner, is full of potential for fanfiction.
One idea could be exploring Birdy's life before she came to Earth. Maybe she had a special mission on her home planet that was interrupted and how that might still affect her actions on Earth. Another could be a 'what if' scenario where Birdy and Tsutomu's relationship took a different turn from the start, like if they were more antagonistic towards each other initially.
Some excellent characters for Birdy the Mighty Crossover fanfiction are Lelouch from Code Geass. His strategic mind and his complex personality would add depth to the story when combined with Birdy. Also, Asuna from Sword Art Online. Her combat skills in a virtual world and her strong will could lead to great team - up moments with Birdy. And don't forget about Ichigo from Bleach. His ability to see spirits and his powerful sword - fighting skills could make for some exciting crossovers with Birdy in different scenarios.
One interesting plot idea could be a crossover with another popular anime where the characters from Birdy the Mighty Decode have to team up with the new characters to face a common enemy. For example, if they cross over with Naruto, Birdy and her allies could join forces with Naruto and his friends to battle a powerful alien threat that is too much for either group alone.
Well, you could start by taking the more angular and sleek design of Birdy the Mighty Decode characters and blend them with the softer, more fluid lines of the other crossover franchise's art style. For example, if it's a crossover with a more cutesy anime, you might soften the edges of Birdy's mecha slightly while still maintaining its overall shape.
I'm not entirely sure specifically what 'birdy novel' is about without more context. It could be a novel that features birds as main characters, or perhaps it has some symbolic meaning related to birds in its plot.