One of the brainstorming sessions we had while in the Discord (and I can make a separate thread on it too, it regarded persistent universe expansion and planet tiering) touched into how you can create flat horizontal progression that was also meaningful, and the answer lies within your arsenal.
Not the Firefall frame (RIP), but rather your collection of equipment. Start off with a base Omniframe and a few weapon variants, and from there you create modifications, you specialize into different areas of combat and utility for maximum efficacy in a desired environment.
You could create a highly mobile loadout that utilizes a lightweight frame and powerful jumpjets, with an assault rifle or shotgun tuned for high rate of fire and consequently short magazine, allowing you to engage, do damage, and disengage quickly.
You could also design another frame loadout to use a sniper rifle tuned for distance, a stealth module, etc. Same flat numbers for all weapons, nothing but horizontal fine tuning, which would become more extreme as the player gained experience and better resources.
Tuning alone doesn't create the standard feeling of progression, however, so among things like visual frame upgrades as your pilot experience increases, we can introduce environmental hazards.
More dangerous and therefore more rewarding planets could have hazards associated with them, making it impossible for untuned frames to visit and establish terraforming colonies. A planet could have an unbreathable atmosphere, requiring pilots to build contained environmental suits (low barrier of entry). Another planet could have a crushing gravitational pull, requiring a powerful frame gravity system (high barrier of entry). These environmental countermeasures could additionally place constraints on the pilot. A frame that becomes more massive due to attached radiation shielding could weigh the player down more, and ECM (Environmental Countermeasures) that pull a lot of energy, such as the gravity ECM, could lower player energy. If you've played Path of Exile, consider these to be negative map modifiers, but planet-wide. And, these planets would have much better loot available to players who take the time to put together properly tuned rigs.
You can accomplish a lot with just horizontal progression, you do not need traditional MMO item tiering or ilevels. You can do material tiers, which would allow for better tuning (further extremes), but that, in my opinion, would be plenty.