I needs some help cobbling a story together

So I have been trying for a while and just cannot figure this story out. I have a few separate snippets of story floating around in my head that I really want to combine, but don't know how. I'll try to list them as neatly as I can and if you can help me at all please do.

I have a villain in mind, he's pretty goofy and isn't truly all that evil, mostly just looking out for himself and doesn't really question where his paychecks come from as long as it isn't for straight up murder. ( all bark and no bite, will threaten someone but wont ever follow through ) He will possibly turn good and join the friend squad

I also want the main cast to be a group of friends with some new additions

I want the main thing to be kinda like a treasure hunt/scavenger hunt/ mystery. Basically, they have to follow clues to solve some mystery or find out a secret.

One of the friends will have been mixed up in some sort of conspiracy against someone in a position of power, but left once it turned from booting them out of office to strait up assassination. ( whether it was a success or not is tbd ) Then some old buddies of theirs from the conspiracy show up and add a pinch of extra antagonist to the mix

I doubt this next idea will fit but I'll throw it in anyway. The friend group travels a train. That's it.

This might be a complete mess but if you can make something out of it them please share!

If you still need help with this (I know from experience that breakthroughs can happen fast), one idea could be this:
So, the one character was nearly involved in an assassination, but left once they found out the truth, right? And if you want it to be a treasure hunt/scavenger hunt/mystery, what about this.
Your villain that isn't actually all that bad could be hired by the conspirator from the (attempted?) assassination to lure the friend to the conspirator so they can kill them for leaving them/knowing too much, etc. The villain, much like the friend, isn't led to believe that the conspirator is actually going to kill anybody. So, your villain leaves a series of clues for the friend, but because they're now in that friend group, everyone tags along for the mystery. The villain could watch and report back to the conspirator for every clue that the group follows. If the friend had left town after the assassination attempt, or if the conspirator had left after being found out, then they could all travel a train to get to a destination for one of the clues. The clues could even possibly be made to reveal weak spots in the group so that the conspirator would better know how to defeat them/separate the friend from the group (after all, his/her original plan was to just get at the one friend). Then, when the group finally follows the clues to the conspirator, and his true plan is revealed, your villain could turn sides and help them escape/bring down the conspirator. After all, he hadn't signed up for murder.
What do you think? Is this kind of what you're going for? I'm totally open to brainstorming with you if something doesn't seem to fit or if there's anything you want cleared up/changed.

WOW this is amazing! It fits perfectly with the dynamics I had in mind, thank you so much!

You're very welcome :) I'm glad I could help.