I love dimension travel, I love crossovers and I love fics where Tim basically gets isekai'd and is being a badass about it so I present to thee: Tim becomes the Shield Hero.
He already has experience fighting with blunt weapons and, while he's a social butterfly when he wants to be, he is practised and comfortable with working alone and he knows feeling ostracised and unjustly mistrusted. He knows what being alone entails, he knows how to survive being hunted.
So despite their protest and despite the hatred he has to endure by everyone around him, he manages to bully the other heroes and anyone who gets on his radar as a fighter to be their best selves and get competent enough to not die and to be able to protect the civilians during the waves.
After a while, his little band of misfits and the other heroes and their people manage to work together and even become friends.
Of course all of that is preceeded by a shit ton of angst and Tim levelling up to an absolute badass.
I also love dramatic reveals, so in this version of the tale, the heroes will eventually be sent be sent back home. No time will have passed at all and, at least in my imagination, they won't have aged but still retain the marks of their quest.
They also get to keep their weapons.
Tim, of course, tells no one, and goes right back to being Red Robin. The others feel like somethings up but they don't have any proof (they have yet to see Tim use the Shield to get out of a tight spot, or be injured enough that he needs to use one of his potions).
Tim kept the book that started it all, hidden in the Nest, but it is found. The family is worried about Tim so, like the sane and normal people they are, they go through his stuff. Thoroughly.
They find the book just as Tim arrives and seeing him tense at its sight, they naturally have to read it.
That's how they all end up in Melromarc.
They appear in the throne room in front of Queen Mirellia, Melty and their guard. The bats don't even have time to brace themselves as Melty barrels into Tim. They're initial defensiveness is replaced by confusion as they watch Tim hug the teary-eyed girl. Their questions get ignored as the Queen quietly talks to one of her guards before coming over as the guard leaves.
They don't know what to think when she greets him, respectful but warmly nonetheless.
Tim knows these people, he's been here before. They just need to figure out where 'here' is.
They have still not gotten any closer to answers when a woman with animal ears rides in on a giant bird and scoops Tim and the girl up.
When the bird transforms into a girl and starts calling Tim 'Papa' (he'd gotten Filo to stop calling him Master eventually and this was the alternative she settled on) the bats have officially checked out mentally.
They vaguely register Tim dragging them through a medieval looking city and meeting a blacksmith? They've decided to just let Tim have his moment and hound him for answers when they get back home.
Why did they end up in Melromarc? Were they summoned? Were the other heroes? Will Tim get to show off the skills he's learned here?
(You could even build in a little scene where Tim mentions how useful his potion skill is with his asplenia and everyone freaks out because what do you mean you're missing an organ?!)