Hey guys I'm writing this editorial as part of my Tips for the Wiki. No doubt many users havean idea of their own personal Ultra but no idea for a good opponent(s). I'm here to help with that, particulary with the aliens.
In the format for enemies you will see that there is a section called History, this is for the Alien's backstory. It doens't matter if the alien is an invader or a victim of an invader or anything inbetween. Alien's (at least the kind we call Seijin) are sentient beings and obviously wouldn't just do anything without a reason. Aliens are often just invaders but there would be a reason for that. In Ultraseven, many of the aliens had unspoken back stories that completely explain their behavior.
The most common reason for invasion is resources, their civilization is facing an energy shortage, overpopulation, environmental crisis, issues we can relate to today. Very few aliens that invaded for the sake of it were memorable monsters though there are exceptions. Note that their backstory doesn't have to paint them in a sympathetic light, the history section is there so users can write down their character's motives in the form of a short biography.
There are also cases when its good for the villain to be two-dimensional. Sometimes the villain is there for the sake of being the enemy the hero(es) has to overcome. This makes sense when the focus is on the main character or to give character development to a supporting cast member. Many of the most memorable Tokusatsu during the 90s like Ultraman Tiga and Kamen Rider Kuuga were actually part of the Drama Genre. Often aliens are used at metaphors for the best and worst of ourselves that we can examine from a safe/comfortable distance.
Powerful but not over-powered
Now obviously your alien has to be a credible threat to be interesting, usually it depends on the story, but if they are too powerful it brings into question the effectiveness of the UItra/hero and makes it more difficult that the next threat would be an actual threat, meaning it would be harder to create a new credible enemy.
Super Ultras exist to fight incredibly powerful foes and only appear for such fights (Noa to fight Zagi, Mebius Infinity Mode to destroy Neo-Ukillersaurus, Ultraman Saga VS Hyper Zetton). It was for this reason that the Three Minute Rule exists, if the hero has a time limit, then no matter how powerful they are most enemies can be a serious threat. However I myself am not in favour of such things and have given my characters a different weakness. In the Ultraman Orion Continuity, the Ultras have a healing factor but in atmosphere environments that is impaired. They are a powerful force but Orion is literally fighting in his Achilees Heel. Be sure to keep these things in mind when next you are creating an alien.