I've been looking at a variety of model-agnostic rules sets for a while now, and I wanted to compile my findings and let others contribute their thoughts.
In this thread, I'll be posting some science fiction rule sets for people to check out.
Xenos Rampant
Publisher: Osprey
Price: $35 MSRP
Dice: D6
Link: https://www.ospreypublishing.com/us/xenos-rampant-9781472852366/
Xenos Rampant is an elegent system based around a set of generic units with fixed stat lines that are enhanced by buying them abilities. This is a low point system game, but can go pretty high in model count and is very friendly when it comes to abstractions. Instead of "wounds" units have "strength", representing the overall state of the unit. An Elite Infantry unit with 5 strength could be a unit of 5 power armored warriors, or a single hero unit, the core unit is the same either way. The Xeno Rules are what give each unit flavor. This game is vehicle and artillery friendly.
Pros:
Cheap to buy, easy to learn, good customizibility, probably solid for house-rules, radically different from Warhammer 40k in its approach
Cons:
Overall unit selection is limited, and there's only so many customizations, radically different from Warhammer 40k in its approach
No Limits
Publisher: Wargames Unlimited
Price: Free
Dice: D10
Link: https://nolimitswargames.wordpress.com
No Limits is a grandaddy an old guard of model agnostic games. Released for free back in 2011 2007 and heavily influenced by 4/5e 40k and VOR, the game has highly detailed rules, extreme customizability, and as it bears repeating, is completely free. The original designer is working on an updated version to modernize the game and step it away from it's roots in VOR. This game is vehicle, artillery, and super-unit friendly.
Pros:
Detailed rules, strong depth of gameplay, absurd customizability
Cons:
Kinda dated and klugey. But if you like that sort of thing, you can call that a pro.
If Worlds Collide
Publisher: If Worlds Collide
Price: Free
Dice: D6s
Link: https://ifworldscollide.com
Link: https://www.youtube.com/watch?v=_h1ldoy9sJU
If Worlds Collide is an in-development open source game system. The game is heavily slanted towards skirmish level play, but can be expanded in to army-level play. The game is very simple to learn, and has some impressive customization options. This game is less friendly for vehicles and artillery.
Pros:
Free + open source rules, community built and welcomes new members to contribute, lots of lists coming out
Cons:
Still in development, really feels limited to skirmish play
Grimdark Future (OnePageRules)
Publisher: OnePageRules
Price: Free
Dice: D6
Link: https://www.onepagerules.com
OnePageRules Grimdark Future is one of the big names in the model-agnostic wargame scene for a variety of settings. The rules are very simple to learn, and if you are looking for a straight Warhammer 40,000 replacement, this is one of the easiest systems to jump in to. The game development is funded through Patreon and sales of bespoke STL files for 3D printing armies.
Pros:
Free rules, LOTS of factions, models available if you like 3D printing, fun and quick to learn and play
Cons:
Somewhat simplistic, limited customizability in units, armies are fixed, game is oriented around a specific size and doesn't scale well while keeping balance
Some systems I've spent less time with but are worth checking out:
Warsurge
Rules are currently free, still seem to be sorta in development. Uses a phone app.
https://www.warsurge.com
Genesys Project
Kickstarter backed rule set by Natfka of Warhammer rumor fame. Game has been very slow in release but looks very polished for what's come out.
https://www.genesysgames.com
Planet 28
Low cost open license skirmish game with rules to adapt to army-scale games. Not great for vehicles, but a fun system.
https://www.mammothminiatures.com/product/planet-28-second-edition