What about using the diversity of Rainbow six Siege in a global conflict like WW1 or WW2. As part of the story you could play as new units like the OSS, SOE, Royal Marines, US Airborne, Marine Raiders, Devils Brigade, Rangers, Philippine guerrillas, Spetsnaz and Alamo Scouts in historic missions like operation Frankton, the Raids on Cabanatuan and Los Banos Prison Camps and set missions in Guadalcanal and Colditz or use movies like the Great escape, Midway, where Eagles dare and others. Also using manuals on hand to hand combat styles like Defendu and Sambo depending on who you play as and new weapons, spy equipment, support vehicles and missions you could create new entries for the Ghost Recon and Splinter Cell franchise. Sniping could be included involving a spotter with unique range funding equipment and load out in both single and two player campaigns you could include missions based on the Black sheep squadron.
For WW1 you could engage in stealthy trench raids using weapons like trench knives with spiked knuckle dusters as you gather intelligence from enemy trenches and return. You could take part in sniping and spotting using camouflage and telescopes and then take part in battles like the Somme, Verdun, Ypres and Caparetto involving chemical warfare on both sides shell shock and graphic depictions of the horrors of war.
This would involve new troops like French, Italian and Russian units using fighting styles and weapons from Savate while Americans would use boxing and wrestling.
Another great setting for a storyline is the Vietnam war as you could play as Rangers, Green Berets, Navy Seals among others or base snipers on legends like Adelbert Waldron of the US Army, USMC scout snipers Chuck Mawhinney and Carlos Hathcock allowing for entirely new gear and customisation options. Also think of famous battles like Khe Sanh, the Ia Drang Valley just to name a few, and the role of air support like the B52, AC130, AC119, ACH-47, Cobra and AC-47 gunships along with dogfights involving A4 Skyhawks against migs and the role of ground support like the m113.
For each game think of the variety of ways to engage the enemy on land sea and air.
For a new experience you could also explore the birth of modern special operations post Vietnam until the modern era using Israeli commandos trained in Kapap and Krav Maga in missions like the Sabena Hijacking, Operations Thunderbolt and Spring of Youth. Spetsnaz troops and FSB could be involved in missions like the Moscow Theatre and Beslan sieges, while SAS, Parachute Regiment and SBS troops could be involved in Operation Barras in Sierra Leone and Nimrod in London, while US Delta Force, Rangers and Seals could take part in missons based Operation Eagle Claw in Tehran Black Hawk Down in Somalia and both The entire spectrum of groups from Anstincould be deployed to the Balkans and Afghanistan to battle the Taliban and al Qaeda as well as hunting war criminals.
For each game think of the way Battlefield one used air land and sea and the way call of duty utilities support options particularly from modern warfare, black ops and world at war