A common theme is power combination. Writers often explore how Ben 10's alien powers can be combined with Halo's technology and combat skills. Another theme is the clash of cultures, as the characters from Ben 10's world interact with the military - focused UNSC and the alien races in Halo.
The most interesting part is the crossover of different universes. Ben 10's alien forms and the Halo universe's advanced technology make for a really cool mix.
There are quite a few 'ben 10 halo fanfic' out there. One that stands out is the one where Ben has to adapt to the rules and combat styles of the Halo universe. The author does a great job of showing Ben's struggle and growth in this new and dangerous world. There are also fanfics that play with the idea of the Halo Covenant encountering Ben 10's aliens, which makes for some really intense and unique storylines.
Well, I'm not sure of a specific 'ben 10 and halo fanfic' off the top of my head. But you can start by looking on platforms like Wattpad or Archive of Our Own. Usually, in these crossovers, the writers play around with how the Omnitrix from Ben 10 would interact with the different races and technology in Halo. For example, Ben's alien forms like Heatblast might have an interesting reaction to the Covenant plasma weapons.
One aspect of 'Ben 10 Evil Ben Fanfic' could be the exploration of the character's dark side. Evil Ben is a fascinating take on the original Ben. It might show how he would use his powers in more malevolent ways, like using the Omnitrix to intimidate or dominate others rather than protect. This could also lead to interesting storylines involving his relationships with the other characters, as they would have to either oppose him or be corrupted by his influence.
I'm not sure specifically as I haven't read this particular fanfic. But generally, it might involve the UNSC (United Nations Space Command) in the Halo universe using the Halo installations in a strategic way to bring an end to a war. Maybe they find a way to activate or control the Halo rings without causing the total destruction they are known for.
A 'Ben loves Gwen' fanfic could explore a different side of their relationship. It might show a more romantic aspect compared to the normal sibling - like dynamic in the show. Maybe it could be set in an alternate universe where they're not related and the feelings develop naturally.
One possible plot could be that Rook and Ben go on an adventure to a new planet in the Ben 10 Omniverse. They encounter a strange alien species that has a unique power. Ben tries to use his Omnitrix to transform into an appropriate alien to deal with the situation, but there are some glitches. Rook, with his Plumber skills, helps Ben figure out what's wrong and together they overcome the challenges and make new friends among the aliens.
Ben 110 can be developed by giving him new challenges related to the Highbreed. For example, he could face a Highbreed villain who has a power that nullifies Ben's alien forms. This would force him to rely on his human intelligence and resourcefulness. He could also develop his character by learning about the Highbreed's language and communication methods, which would show his adaptability.
I haven't actually read this specific fanfic, but generally in a 'Ben x Esther' fanfic in the Ben 10 Omniverse setting, it might explore their relationship in different ways. It could involve adventures where they team up, and their characters' growth while dealing with various threats in the Omniverse. Maybe it also delves into how their different personalities interact and complement each other.