If you want the game to be harder you need to modify the Establisment and Conquest stages. It's too late when you're in the End Game stage (unless you screw up big time).

Establisment stage: each faction is just starting. They have small armies. There are various rebel settlements nearby. Fighting between different factions is normally over small parts of their Kingdoms. This stage ends when the various factions try to conquer one another.

Conquest stage: each faction has established borders. They have medium to large armies. All the rebel settlements and weak factions have been crushed. Fighting occurs between factions trying to conquer each other and enlarge their Empires. This stage ends when the strongest 2-3 factions attack each other.

End Game stage: one faction has emerged as the strongest in the world. There is no faction left to challenge its power (though it may have troubles with internal rebellions). Fighting occurs between this faction and everyone else. This stage ends when there no more land to conquer.

OP since you're in the End Game stage, all that's left is crushing the remaining factions. Giving ceasefires to the Danes and Byzantines is pointless since they won't be able to build up their armies enough to be challenging (it's too expensive for them to create and upgrade an army large enough to have a chance against yours). Also it's very unwise to attack a faction that's much stronger than your factions.

If you're bored just conquer the remaining factions, control everywhere in the game, then spend the rest of the game building up your Empire (improving cities and castles) until you can fight the Mongols, Tirmids, and Aztecs.