The base premise is people colonising the surface of Mars. I've found theoretical means of creating an Earthlike atmosphere and magnetic field, but the one problem I haven't found any solution to is gravity. There is no way to "increase" Mars' gravity without using technology beyond what would be available to my colonists - there was a lot of mention on Quora of basically needing to be able to shred a nearby planet for additional mass because there aren't enough asteroids in the whole solar system to "bulk up" Mars enough to give it Earth gravity. Short of handwaving it with a magic gravity machine, the solution I came to was transhumanism. My questions are these - what are the negative effects of living in low gravity and what kind of implants/augmentations would make life in Martian gravity possible? If it's gotta be invasive and horrifice then, you know, cool. Thanks!
The only detriments I can think of from living long term in low gravity would be loss of muscle mass, bone density, and cardiovascular fitness. All of these would come from the body not experiencing the same stresses as it would under Earth's gravity. Astronauts who spend time on the ISS already experience this, but since Mars is a low-gravity environment instead of a zero-gravity environment, the effect would probably be far less pronounced. I'm not sure you need any implants or augmentations to deal with that, just an intensive exercise regimen. People might have implants that track their vital signs, blood oxygen, bone & muscle mass, etc. like a super-futuristic Fitbit, but I don't think you'd need much else.
Off the top of my head, things like bone mineral density and muscle mass aren't going to be huge issues unless your people are going back to earth some day, or have to be physical labourers. I suppose they would technically have and increased fracture risk if they were hit by a car or something like that as gravity plays a minimal role in generating fractures in those situations, so the decreased bone density would be a detriment in those cases.
If you want a mundane solution to the bone density and muscle mass problem, just have everyone strap bungee cords on themselves for a morning run everyday to produce the impacts required for adequate BMD and muscle mass. Reasonable medical solutions are to use a bisphosphonate or teriperitide for bone mineral density and then make up a new drug that prevents down-regulation of muscle mass after exercise. If you want an exotic solution then use genetic engineering to have them modify the genes that are master regulators of bone and muscle homeostasis to shift the balance towards increased BMD and muscle mass.
Long term, you're probably going to have to be more worried about ionizing radiation than gravity, but you said you have an acceptable solution to that (I would personally use a molten salt battery in the center of the city to generate a magnetic field, but I'm uncertain how big it would have to be to sufficiently deflect charged particles). With a magnetic field you'll still have to worry about ionizing neutron and gramma radiation, but presumably your atmosphere takes care of that. If you're really persnickety, you'll want to determine what your atmospheric pressure is and what its composition is. You only need about 0.1 bar of oxygen to be safe for humans, and honestly can probably cheat this down lower if you needed to by having people acclimatize to it over time. The volume of gas required to produce an atmosphere of 0.1 bar is exponentially lower than that required to produce 1 bar of atmosphere and equivalently would have exponentially lower shielding effects from gamma and neutron radiation from the sun.
Equally with gamma and neutron radiation, you can solve it by having an enormous atmosphere, have everything shielded by water, making up a new medication that condenses in the skin and preferentially absorbs gamma radiation (neutron radiation may need a different solution) and turns people gold colored (because it's cool), or make up a genetic engineering solution to either produce a radiation absorbing protein, or more accurately repair double stranded DNA breaks to avoid the adverse effects of being exposed to radiation.
Reduced gravity may also decrease blood drainage from various parts of the body including the sinuses in the brain, which can increase the risk for blood clots. We don't yet know how pronounced these effects are (because the first one was only picked up in an astronaut a few years ago) but it is probably something that would be noticeable at a population level in the form of increased strokes and increased pulmonary embolisms. You can address this by having everyone take a blood thinner daily(aspirin if your lazy, apixiban if your fancy, or tinzaparin if your old school). Or if your genetically engineering your people than just add that to the list of things that gets fixed.
If you decide to genetically engineer your people that adds another layer of things that can go wrong in the form of unintended gene combinations that produce either a reduced effect, an amplified effect, or a completely unanticipated effect. If you engineered muscle mass you could anticipate that some people in the second and third generations may end up with low muscle mass because they didn't inherit the modification properly, others will have double or triple muscles because of an unexpected interaction with some other gene that wasn't known about at the time of modification, and others will end up with hypertrophic cardiomyopathy and have sudden cardiac death at a young age because their heart unexpectedly grew way more than it should and stopped being able to pump effectively. The same sorts of problems would come up in the other engineered features like radiation tolerance, increased bone mineral density, or blood clot prevention.
More than you asked for, but have fun!
I have an idea. I have to warn you that this solution is literally as dumb as a box full of rocks.
In order to give the muscles more to do, strap on some extra weight. A really simple way to do this would be to wear a backpack stuffed with whatever high density rocks are available.
Those wishing for a more intelligent sounding and elegant looking solution could have tungsten plates sewn into their jackets.
I suppose if you wanted to make it more complicated, invasive, and horrific, people could implant rocks and metals into themselves to increase their weight. Think piercings taken to nightmarish levels.