Here's some ideas to improve on the mod, maybe something is viable and you would like it:
Stat attributes like S, A, I or S, S, D, W meaning:
S: Strenght
A: Agility
I: Intelligence
or
S: Strenght
S: Stamina
D: Dexterity
W: Wisdom

Those would be added as main character resources that would define:
melee attack power (Strenght)
movement speed bonus (Agility or Dexterity + Stamina)
HP (Strenght or Strenght + Stamina)
Crafting Speed bonus (Intelligence or Wisdom)

The other stats would be accessible by skills in which you would be able to spend points via Level Up. The main attributes scaling would be fixed, so the stat progression would be the same for everyone, but say we need a dude that can carry a lot or someone who loves handcrafting stuff, those things would be accessible via Skill points that you earn when you level up.

So on the skill part we could have something like:
- Melee power (%bonus to damage with picks)
- Gun power or Marksmanship (% bonus to dmg with guns)
- Heavy Weaponry or Big Eddie (% bonus to dmg with explosives)
- Chaffeur or Driver (% Efficiency bonus to vehicles, in which makes them use less fuel when you ride them, or gives a resistance bonus to manned vehicles)
- Craftsmanship or Builder (Crafting speed bonus)
- Logistic Commander or BotBilly (Logistic trash slots and logistic request slots bonuses)
- Hauler or DonkeyKong (+ Inventory slots)
- Technician or Mr. Spock (Bonus dmg to laser weaponry, enable railgun to add bonus dmg to railgun)
- Army Commander (Weapons of the vehicles manned by this player will have a % bonus to dmg)
- Rambo or Mounted weaponry speed (Weapons of the vehicles manned by this player will have a % bonus to shooting speed)
- "I'm BACK!" or Heavy Mounted Weaponry (Weapons of the vehicles manned by this player will have a % bonus to explosive dmg)

I know some of these stuff might not be viable to implement, bcoz LUA. But something else could give you ideas on how to improve the mod, I'm also thinking on the possibility of using this mod with AAI mods for a RTS style where you upgrade your troops with this mod's Idea, would be interesting.

Look awsome! Thank you for the ideas!
But I think there is a problem to do that with factorio.
I think damages are calculates exclusivelly by the the weapon + ammo. There is no character variable to increase damage.
I maybe wrong, but I think there is no way to do that.

The options I have related to a character:

character_crafting_speed_modifier
character_mining_speed_modifier
character_running_speed_modifier
character_build_distance_bonus
character_reach_distance_bonus
quickbar_count_bonus
character_inventory_slots_bonus
character_logistic_slot_count_bonus
character_trash_slot_count_bonus
character_maximum_following_robot_count_bonus
character_health_bonus

I believe that 0.17 will help providing the other modifiers into the game.
Heavy tank mod adds new weaponry with custom bonuses for those weapons, I believe you can get the intel to add bonus upgrade with that. Bob's warfare also adds weapons with dmg bonuses per weapon, not sure if you can learn something that can help, I'm not sure.
But the HT mod could suit it. If you could change the skills on the gUI to "technologies" that doesn't show on the tech tree but are instantly completed when someone learn points on a skill. Downside of that is that it becomes a force-wide buff instead of player-only buff.

Apart from those ideas, the XP seems to be oddly calculated or some event might not be working fully, because I'm murdering hundreds of behemoth biters on lvl 21 xp but my xp isn't filling up, but when I complete a research, it seems that the research xp and biter xp are added at the same time so my almost empty bar before a research is completed gets filled instantly with around 5~10% on the next level after completing the research.

Wisdom
character_build_distance_bonus
character_reach_distance_bonus
character_logistic_slot_count_bonus
character_trash_slot_count_bonus
character_maximum_following_robot_count_bonus

Strength
character_mining_speed_modifier
character_running_speed_modifier
character_inventory_slots_bonus
character_health_bonus

Dexterity
quickbar_count_bonus
character_crafting_speed_modifier

If you can give damage bonuses then that would fall under dexterity. Grenade and bullet damage.

Dunno just suggestions from an TRPG nerd who likes fidly character bits.