Bluntly, that would be hideously boring. We're trying
not make a UNSC reskin here. Personally, I think that it'd be much more interesting if we explored more of the alliance's genetic engineering innovations. Like for example;
In the 22nd century, manipulation of the human genome became commonplace. Techniques of genetic engineering advanced to the point where the rich could custom-build fetuses that grew into stronger, smarter, and more attractive adults. In more permissive regions, custom-designed life forms and 'uplifted' animals occupied an ill-defined niche between 'property' and 'sapient being'. Travel to planets with unique forms of life brought an awareness that Earth's unique biodiversity could be lost if spliced and hybridized to gain useful alien qualities. The Sudham-Wolcott Genetic Heritage Act was passed by the Systems Alliance Parliament in 2161. It imposed sharp restrictions on controversial uses of genetic engineering, but provided government subsidies for beneficial applications.
SCREENING AND THERAPY: Most governments provide free assessments and corrective therapy for genetic diseases in prospective parents. This has nearly eliminated everything from cystic fibrosis to nearsightedness. The earlier screening and therapy is performed, the more comprehensive the results. Though ideally performed on artificially fertilized zygotes in a lab, procedures are available for embryos in the womb and newborns, out of respect for personal beliefs.
ENHANCEMENT: Improvement of natural human abilities is legal, but adding new abilities is not. Treatments to improve strength, reflexes, mental ability, or appearance are permitted; adding a tail or the ability to digest cellulose is not. Some genetic enhancement is provided for free to Alliance military recruits, but the average citizen must pay for the privilege. The process can take years to reach fruition in an adult.
ENGINEERING: Artificial hybridization of genes from compatible non-human species with human genetic code is illegal. Creation of designed life is broadly legal (and mainly used for terraforming and medical applications), but sentient creatures are heavily regulated, and creation of sapient life is outlawed by both the Systems Alliance and the Citadel Council.
So things progressing further down this path to the point of some nations uplifting certain animals, creating human offshoots, or even just massive advancements in cybernetics would be pretty cool.
On a wider note, having humanity encounter other aliens would open the doors to alternative paths. The rachni at least, managed to scatter so many damn savior ships that Binary Helix managed to stumble upon one and experimented upon the queen. A rachni war would probably be a massive source of divergence points - especially if they managed to escape into the wider galaxy - or end up joining the Systems Alliance as another member.