I really hate the classic client visual style when playing Interlude. Is there any way to make the game look normal without triggering anti-cheat or something?

I suppose it could be theoretically done, but it needs to be specifically tailored to the client the server you're playing on, and afaik these "classic interlude" are actually running on a modern client with only gameplay "downgraded" to Interlude = someone would need to create the "old" interface from the scratch. But I'm definitely interested too, I feel like the classic interface takes away the oldschool feeling of the L2

Don't forget protocol version and any anti cheat method being used.

You could always PM the Admin and find out what he thinks. Getting their permission might prevent you from getting a ban.

Plus then you also have to pay attention to any updates, just in case the main L2.exe is modified.

Who knows maybe the admin will like the idea and you could both find a client that not only looks good but runs as well.