Difference with default faction:

• 50% HP
• -100% kinetic resistance
• 99.99% explosive/artillery resistance
• 99.99% poison resistance
• Short ranged attack
• High resistance to 10+ range damage (simulate dodge)
• Flying
• Ignore walls
• Can't produce juggernauts (flying bitters should be designed to die fast)

Reasoning:

• They take fire on themselves to allow other bitters to crush megawalls
• Enable bitter faction option of flankiong: unit which can burrow(in this case - fly) through walls and attack factory/turrets from flanks, bypass killing zones entirely as tactical option, not strategic one

P.S. If you really want to make Juggernaut, make it carrier from SC2

Unfortunately, a full-fledged flying faction is currently impossible, because. pheromones do not propagate through water/void. The idea is taken into account, but first I need to see what will happen in factorio addon - they promise flying squads

I think you will like the new faction in the almost finished release - "arachnids"
They don't fly or ignore walls. But defense without machine gun protection will be destroyed by them - lasers and flamethrowers, without sufficient concentration, will not be able to kill the eggs spawned at death in time and the flow of respawning biters will be endless )

Can I enable new factions without game restart?

P.S. Not sure it intended gameplay, but my friend still defending base with only megawall and machineguns, even without rockets; ~35 hours in game, tier 3-5, 200% spawn rate and area; SE2+K2+Rampant Arsenal.

Hmm.. Maybe then not flying but "gliding" faction? Move and group like any ground bitter, but make them fly after they found target for attack? It will accomplish mission of "jump through wall" at least.

I am not sure what use of pheromones is and how bitters AI is written. What should I read to better understand?

Yes, the faction is active by default, any "base" can become this faction when mutating.

For general understanding, simplified:
In rampant, each chunk contains a number (and not just one), with an estimate of its attractiveness to biters. This number is called "ferromones" of the player's base. With distance, this number decreases.
If the chunk is not passable, then everything beyond this chunk does not contain pheromones. In such a situation, the AI ​​cannot determine where to go. This way, the flying units will behave like normal units, following the buildup of pheromones.
Support for flying units requires a rewrite of chunk evaluation algorithms.

This work will eventually be done, because comparing the assessment of the usual path and the flight will provide qualitatively new opportunities for AI, but this is a matter for the far future.