Yes, there are likely to be such fanfics. Fanfic writers often explore unique relationships in their works, and the idea of Marcel calling Klaus 'dad' can add an interesting dynamic to a story. You can try searching on fanfic platforms like Archive of Our Own or Fanfiction.net.
The plot could involve Marcel having a crisis where he realizes that his biological family has let him down in some way, and Klaus, who has been there for him in many situations, becomes the person he looks to as a father. Maybe Marcel gets into trouble with other vampire factions, and Klaus is the one who steps in to save him, leading to this newfound father - son relationship.
There were many other books like "Poor Dad, Rich Dad" and "The Weakness of Human Nature".
Reminiscences of a stock operator
Wisdom of the stock operator
The Little Book That Beats the Market
4. The Art of the stock operator
5. The Intelligent Investment
The Most Important Thing in Finance
The True Nature of the Financial Market
The Economics of One Person One Company One Market
The Little Book of Common Sense Investment
10 The Art of Stock-Manipulator 2
These books are classic works on investment and financial markets that can provide readers with knowledge and skills related to investment and financial markets. The readers could choose books that suited their interests and needs.
In the comics, Klaus is not immortal. He may have certain powers and abilities, but there are limitations and factors that can pose a threat to his existence.
In some comic versions, Klaus is indeed depicted as gay, with storylines exploring his romantic relationships with same-sex characters. However, this can vary depending on the creative team and the direction of the story.