Well, he could team up with Superman. Imagine Ben using his alien forms to help Superman fight Lex Luthor's latest evil plan. His alien powers would add a whole new dimension to the battles in the DC Universe.
The unique aspect of 'ben mari dc fanfic' could be the combination of characters named Ben and Mari in the DC universe. It might explore relationships or storylines that are not part of the official DC canon.
One common theme could be adventure. Given the DC universe setting, stories might involve Ben Turner and Vixen going on various exciting quests. Another theme could be the exploration of their powers and how they interact with each other in different situations.
Another theme could be redemption. If either of the characters has a troubled past, the fanfic could explore their journey towards redemption. For example, if Ben Turner has some past actions he's not proud of, his relationship with Mari could be part of that path to making amends. Also, a theme of discovery could be present. They could be discovering new powers or aspects of the DC universe together.
One idea could be Ben Tennyson getting transported to the DC universe and his Omnitrix's powers attracting the attention of various female characters. For example, Wonder Woman might be intrigued by his unique abilities and his sense of justice. Another could be that he teams up with some of the female superheroes to fight a common enemy, and during the process, a harem - like relationship starts to form.
Well, there's 'Ben 10 and the Justice League'. In this fanfic, Ben teams up with the Justice League to fight a common enemy. It has great character development and a really engaging plot. Also, 'Ben 10's DC Adventure' is quite good. It takes Ben on a journey through the DC world, facing off against various villains while making new friends among the heroes.
One idea could be that Ben, with his Omnitrix, somehow gets transported to the DC universe and meets various female characters. Maybe he has to team up with them to stop a common threat, and during the adventure, relationships start to form. For example, he could work with Wonder Woman, and she is initially skeptical of his powers but grows to respect him.
I'm not sure specifically which 'ben mari dc fanfic' you might be interested in. There are many fanfic platforms out there like Archive of Our Own (AO3) where you can search for such fanfics using the relevant tags.
His powers might be enhanced. In the DC Universe, there's a lot of energy and power sources. Maybe the Omnitrix could absorb some of that energy and give Ben new and stronger alien forms.