1. Set it in the near future, 2040-2055ish. This gives enough leeway with the audience to establish the beginning of Weyland-Yutani and mankind's attempts at interstellar travel but doesn't put it so far ahead that slightly updated versions of (then) current technology become dated.
2. FLESH OUT THE CHARACTERS. The only elements existing character-wise that rush to my mind are the brothers' dynamic (Dallas watches out for Ricky, even if it means jail time) and the Sheriff's "by-the-books" approach. Amplify that. Don't have Dallas just go "hey Ricky, let's get your keys," show him actively trying to keep his brother out of trouble by taking the heat. Kelly's alienation from her husband and daughter got maybe one scene to develop, and the timing was perfect to have this amplified by possibly having PTSD. She saw combat during the Iraq/Afghanistan era and this new threat is the one time she feels "at home" back in her town.
3. Contrast the lighting more. It being pitch black was the right choice for certain scenes but not for the whole thing.
4. Maybe it's just me, but I think the scenes where the Predalien tears through the Yautja ship and the ensuing "call" to the Wolf should have been shown in glimpses during the opening credits rather than given full screen-time. Think how Alien 3 immediately set the tone by relaying the events between listing the cast and crew.
5. Don't show so much of the Predalien, leave most of it as a mystery. The hospital impregnation scene does this wonderfully, but the rest of its scenes come across as gaudy and schlocky.
6. One of the reasons that the action scenes in Aliens (and, I'd say, most of Cameron's work overall) stand out so well is because of variety and pacing. If you're going for a more action-focused approach, then figure 3-5 key moments and how to make each stand out as unique. If an audience is given gratification at no cost then the prize is worthless, and if all the action is the same then none of it truly stands out.
7. Don't change that ending (after the helicopter crashes) one bit. I LOVE how AVP:R ends, giving us hints at how Weyland-Yutani become a conglomerate and focus on developing interstellar travel. Anything that gives Robert Joy more work is alright by me, as well.
8. In Aliens, the USCMC explain that the Pulse Rifle and Smart Guns fire explosive-tipped ammunition, and we see during the ventilation shaft chase that Vasquez's .45 rounds do very little damage to a Warrior. It never sat well with me that standard NATO 5.56 rounds would do the same damage.
9. Remember what made the original work. It was a b-movie concept given a-level care. Stop, think about each decision being made, and then wonder if it sounds like a Charles Band feature.