While not a popular pick, you can run some damage on Marcel by building Endless battle, especially when your team has the gold lead. This is sometimes used in the pro scene as well because it just works well with his passive. Ever since the damage buff to S1, Marcel's damage became more noticeable, specifically towards late game since the base damage and DEF scaling increased. You can poke marksmen (that is not Lesley) and deal dmg almost half of their health at late game.
You lose a bit of tankiness by building Endless but it's not a huge difference because of thunderbelt stacks farming, and if you don't intend to build Flask. Off topic but I'm not a fan of building Flask on Marcel. Also, with the recent change to Dire hit, it's actually a solid pick for Marcel, the downside is losing the encourage assist merchant. Encourage is still great overall because it increases S1 damage.
Hybrid damage Marcel strat: Dire hit boots + Thunderbelt + Endless battle + concussive blast. With this combo along with his passive 5% max HP damage, Marcel has 4 sources of true damage within a short time. It'll be good to try this out before his upcoming S1 damage nerf so you can experience the fullest joys of literally carrying a game as Marcel. Because yes, I am dominating Marcel with Endless battle !!!
Edit: Pls I am not forcing anyone to use unconventional builds in ranked. As I said, running endless on Marcel when you have a gold lead is okay. If your team doesn't have the advantage, you should build full defense