I was picturing DC 2.0 being fantasy but where all the fluff is that the PCs are High Tech. That being the case, rock on d20. Rock on. Displacer Fields, Force fields and Invisibility are already well covered and portable to d20.
So d20 seems like a fine system since you do want your marines to crunch through lesser foes, and you likely aren't as interested in caring about whether mooks can still succeed on a lucky roll.
If you like Frank's notion that one-shotting dragons should be a thing, then you can still get by with massive HP attacks that cause massive damage saves. Perhaps re-tweak the massive damage rules/saves some and you'd be good to go there. I'd think that stronger dragons would not expect to be felled in 1 shot however, since that's a silly design goal.
The only downsides I see for d20 are that handling combat with large amounts of opponents doesn't go smoothly, and skill system. I think the skill system has less weight thankfully since a lot of abilities will be binary, whether you have proper tech or not.
A possible notion: ratchet down damage and hit points. Like seriously, divide by 10. If something can't deal/take 10 damage then it can fuck off. Elder Earth Elementals get 22 Wound Points; Blast him! This could help in speeding up combats to allow for more larger amounts of opponents too.
For the diamond axe vs. ballistics concern, well there isn't a good reason why those two weapons should be comparable. Ballistics have range, often can cover an area (for automatics or energy weapons), but you have limited ammo and in close quarters fighting ballistics may not be as desirable. Perhaps implement a parry mechanic which makes ballistics much less reliable in close quarters if AOOs aren't enough disincentive. That and some weapons can be countered or ablated by energy resistances/immunities, and incorporeals and such are immune to bullets. It is not too hard to create valid reasons to use melee and ranged tech weapons without resorting to soak and dicepoolery.
Now with the apology for d20 aside, is there room for something like Tyranids/Gene Stealers, or do those kind of go with the Warrior Race? Get some biotech jazz going on.
Docs n' Engineers: Not needing a healer but instead using stim packs is a feature, not a failing. I always thought the doctors on star trek were a complete waste. I could see an engineer being viable and happily so. Their unique schtick being repairs/craftguyvering and minions/mechs.
All engineers would be able to repair/modify stuff. Maybe you want to turn that machine cannon into a sniper rifle, or that laser defense grid into a hologram projector, or that biomass into a semi-sentient wheelbarrow. Different strokes for different origins, but they are all experts in keeping equipment in a state of proper repair, especially for their origin.
| origin | Engineer | schtick |
| Deathworld | Adeptus Mechanicus | servo skull, gives equipment boosts |
| Warrior Race | Brood Queen | hatches biotech like zergling bombs |
| Mechanical | Drone Hive | robot swarm |
| Ubermensch | Operator | robot minion, portal pistols |
| Daemon | Warp Priest | boost equipment with infernal, summon demons |
[edit: post needed tweaking. A dozen disjointed 1 sentence paragraphs was kinda hard to follow. some thoughts added]
