I have been trying to grind out a TOS character exclusively in the 23rd century and had a few ideas on how this part of the game can be expanded for affectianos of the original series. First step would be to expand the TOS map to include the Risa, Kassae, Xarantine, Aldebaran, Donatu, and Archanis sectors.
Patrol missions easily ported from 25th century to 23rd century. Most of these either involve the Naausicans, Gorn, or Orions as enemies which still works in the 23rd century, or they involve scanning objects without combat.
Teneebia Sector: Icarti System, Elvren System Patrol
Rator Sector: Chulan System Patrol
Vulcan Sector: Beytan System, Bhea System, Pico System Patrol
Argelius Sector: Europani System, Kei System, Pellme System, Ruben System, Xleen System Patrol
Orion Sector: Kinjer System, Reytan System Patrol
Celes Sector: Mandel System, Maro System, Veela System, Watran System Patrol
Risa Sector: Donia System
Kassae Sector: Cernan System, Dace System, Sandah System Patrol
Xarantine Sector: Donteri System, Honod System, Seedea System Patrol
These missions could be ported over, but require some work or explanation:
Eirhees System Patrol: Remans, still workable, but need a justification for why they are in violation of the Neutral Zone
Servin System Patrol: Orions salvaging a Borg Sphere. While the Orions are fine, the Borg are out of place in the 23rd century.
Khoolhaas System, Omar System, Tayi System, Kern System, Una System Patrol: Klingon ships which would need a 23rd century make over.
Storyline missions reworked for TOS characters operating in the 23rd century:
Everything Old is New, Night of the Comet, Past Imperfect, Core of the Matter, Terminal Expanse
Add a 23rd century Deep Space Station K-7. The PvP tutorial missions with the exception of Kerrat Warzone should be accessible from this location as TOS is already as subset of Federation faction. A vendor on K-7 could sell Klingon TOS disruptors.
TRIBBLE a 23rd century Drozana Station. As well as being the launch point for the Specters missions, it should also have a vendor selling Romulan, Vulcan, and Andorian plasma weapons.
23rd Century ESD should have bank and mail access. There is no reason why items deposited by a TOS character in the 23rd century would not be able to reclaim those items after being shunted 200 years into the future. I would think that temporal investigations would be aware of the character's need for them in the future. The consoles between Admiral Garrett's office and the Transporter work for this.
23rd century ESD should have a DOFF contact as well as consoles for doing lore missions. The TOS characters are temporal agents after all, and will need to know this stuff, even before being transported to the future. The Yeoman outside of Admiral Garrett's office can work as the contact for this.
Ships for 23rd century LTCDRs and CDRs:
Add to the shipyard the Refit Connies for LTCDR characters. Possibly add a regular non C-store(-1 Tac console slot, change Lt univ station to Ens Sci) Daedalus class science ship as well so players can choose between either a cruiser or science ship.
Add the Constellation class cruiser (non C-store, disable Dakota, Cheyenne, and Stargazer options until after Battle of Caleb). Add the Horizon class science ship and disable Olympic and Hope class options. The canonical explanation for these would be that the ship is the second or third commissioned of its kind.