There are a few DLCs I would like to see -
All the Way to Timbuktu - This would expand the map further into Africa, adding more provinces in Mali & Ethiopia as well as adding Kanem Bornu and part of the Swahili Coast, where you can form the Kilwa Sultanate. There would also be new trade routes going from Mali to North Africa as well as trade routes from the Swahili Coast to the Middle East & India (and if there is hypothetically a China expansion soon after, to China & Indonesia). Of course, much of Central Africa would be wasteland.
Rise of Islam - Before anything else, I want to address something. I know that in Islam it is a major sin to depict Muhammad. However, I have scrolled through House Umayyad & have discovered that Paradox has already created a symbol for him - the standard piece of calligraphy used normally. Now that that's out of the way, Rise of Islam would add four new start dates - 609, when Muhammad began preaching Islam, 632, the death of Muhammad, 640, near the beginning of the Arab-Byzantine Wars, & 750, the rise of the Abbasids; however, you could start playing from 600 if you so wished. There would be new religions - Semitic (to represent pre-Islamic Abrahamic beliefs in the region), Chalcedonian (to represent pre-Schism Christianity, though it would exist in the start dates until 1084), & Unified Islam (to represent pre-Sunni/Shi'ite split Islam). There would also be changes to Zoroastrianism, the Coptic/Miaphysite Church, Manichaeism, (which would become a separate religion from Zoroastrianism), and early Islam.
Glory of the Renaissance - This would essentially be the opposite of RI - while Rise of Islam would take the timeline back, Glory of the Renaissance would take the timeline forward. The game would end at 1520 and there would be four new start dates - Rise of the Ottomans in 1444 & Fall of Byzantium in 1453 (both of which are already in EUIV), as well as Wars of the Roses in 1455 & Reconquista in 1492. The expansion would also introduce a cadet system, the advent of gunpowder, an enhanced Black Death, and events to simulate the conquest of the New World which would give you ducats, prestige, etc. (Unless Sunset Invasion is enabled

). You would also be able to fund artists which will cost you ducats but will give you additional prestige.
Vikings of Vinland - This would expand the map west towards Greenland, Markland, & Vinland (to about Nova Scotia, maybe New York). There would be many new cultures & religions for the Native Americans, as well as many new Norse events & decisions. There would also be a new bookmark - Journey to Vinland in 1000. Of course, this would probably work better as a mod, but if it is a DLC, only Greenland would come with the patch - the New World would be DLC locked for those who don't want North America in their game.