Egosoft kinda indirectly admitted that they are working on Boron, so to full classic set we just missing the Split. For "less classic" we also miss Terrans (my personal favorites).
Here are my ideas for various DLCs and their key features:
1) Boron and Split Big DLC - as these two were always enemies it seems natural to have DLC where we got access to Boron and Split terriories locked in perpetual conflic with eachother and campaign based around it.
- You could support Boron in their goal to construct chain of Super Forteresses that would break Split will to attack (award - construction plans for Super Fortress).
- You could support Split in their goal to buil massive super destroyers to steam roll Boron defences (award - construction plans for Super Destroyer).
- You can build reputation with both and broker peace solution (you will be awarded with Super destroyer and SUper Fortress construction ship, but not blueprints).
- You can exploit the conflict as Pirate (award - loot!).
- You can use your late game might to defeat both and enforce peace (award - making you late game fleet useful for something

- new Boron and Split starts.
- military career mechanics
2) Terran Big DLC (Sol, Aldrin, Sectors/systems east to Aldrin including heavily infcted Xenon Sector, plus the sector south to it which would connect entire region to base game Universe.
- Sol and Aldrin connected by Terran build gates.
- Terran USC and ATF merged and no longer separate fleets.
- Aldrin become fully Terran territory, but there are still some unique Aldrin ships for local forces.
- Story is "fight" and "build" about Terran effords to "push Xenons east" after Aldrin gate suddenly reactivated - mix of military missions and well as build/supply/logistic mission for Terran stations and outpost build after gradual push out of Xenons.
- Unique Terran equipment - jump drives: a) Normal Terran Jump drives act like X-Rebirth capship jump drive, but without the need for local jump beacon. b) Advance Terran Jump drive - allow gateless jumps between systems without the need of Gates, albeil at big fuel/energy consumption and long cooldown (cannot be skipped, unlike in Rebirth where player could used "coolant"). Normal jump drive is locked behind high enough reputation with Terran. Advance jump drive is locked behind completing Terran campaign (you can buy the Adv JD after plot).
- new Terran and Aldrin starts.
3) Xenon and Khaak Big DLC.
- new ships for Xenon and Kha'ak
- more focus on Xenon economy
- new game start - corrupted Xenon AI - you start as corrupted Xenon fighter AI that gained sentience - you have to survive and upgrade (scrap, or your secretely build stations) your CPU/Memory without getting attemtion from local CPU ship - the CPU ship is your overlord that give you missions and directives, but if it notice you're corrupted then will try to purge you.
- Xenons are friendly (unless you pist off the CPU ship) and you have access to Xenon economy.
- All other races are hostile.
- your goal is to become the CPU ship yourself either by secretely ungrading and building your own CPU ship or by hacking/takingover your overlord CPU ship.
- teleporter device is replaced (you do not own physical body) by "consciousness data transfer" so you can freely teleport between ships that you own.
- new game start - Khaak Queen - you start as Khaak Queen aboard stranded Khaak mothership. Mothership arrived in local space by miss-jump and is capable of mineral processing, ship, station construction, hatching of new Khaak NPCs. As Khaak Queen you're physically stranded inside Mothership, but as Khaak are known by telepatic abilities (at least what I remember from X2) you can "assume direct control" of other Khaak in your swarm. Your objective is to find the way back to Khaak space by obtaining various Khaak artifacts left after X3:TC war as well as surviving hunts from the other races that spot your activity.
- other races are hostile.
- new campaign (if you choose not to play Khaak or Xenon) - NPC Khaak mothership is spawned and it tries to build it's forces for invasions (areas where the artifacts are stored). You are contracted by "normal races" to hunt down the Mothership ("Think" and "Fight" by solving various puzzle and exploration quests kinda like old Treasure hunt campaign).
4) Gateway small DLC (one new sector and one "unique" new system).
- new campaign - you are contacted by the group of scientist, who discovered new sector with aparently malfuntioning gate.
- that malfunctioning gate work in the way that it connect to randomly generated system (kind like Unfocussed Jump Drive sector from X3, but entire system scale) - after pre defined timer the gate loose the connection - system and all ship inside are purged from game - if you are inside the system - you're dead.
- the aim of campaign is supporting the scientist with gate research. Intially you help them build research station near the gate (simble build logisic missions). Then you will be given varios missions:
a) Exploration mission - you're sent into the new systems for exploration, scans, artifact recovery etc.
b) Escort mission - you escort scientist research ship on a trip inside the system.
c) Expand research outpost.
d) Enemies at the gates - there a chance that new system connect to Xenon or Pirate infested system and you have to hold the gate and defend research outpost untill gate reconnect again.
- after time with these missions you will be able to extend the time between gate reconnection (and in the same time expand the generated system as initial "short loops" will require that only small area to be generated).
- ultimate rewards will the "gate control" and "gate stabilisation" where you will be able to force gate reconnect (to generate new system) as well as stop the timer (e.g if you find the system you like you can keep it as your private system).
5) Black Flag small Pirate DLC.
- adding various pirate clans and Pirate unique ships.
- expansion of pirate economy and behaviours.
- ability to employ pirates (e.g. to sabotage your competitors).
- unique pirate/boarding equipment.
- new pirate game starts.
- campaign to become famous pirate (story campaign).