The answer is obvious: the empire of Alexander the Great, as of his OTL demise. (This presupposes I'd actually be obeyed by his generals etc., of course, which is not realistic.) Since I have a pretty solid grasp of both paper-making (having once worked a summer in a historic paper mill), how crop rotation fuctions, of how a blast furnace works, and what a heavy plough is... I think I could do some interesting things. (Note to self: look up the correct formula for gunpowder, and learn how to make it.)
Some things I'd implement:
-- The establishment of elementary schools for all citizens in every city and town. The most talented would we sent on to receive further education (either in a particular craft school, or at a proto-university). At the very least, the entire population should be literate.
-- The establishment of libraries, combined with said proto-universities, to serve as repositories of knowledge. All of these imperial libraries should ideally have copies of all works entered into the, ah, 'library system', so that the accidental destruction of one library still loses us no knowledge at all. The high literacy rates and the availability of paper should make this feasible.
-- Such university centres should also include medical schools and hospitals. Practices such as boiling all drinking water (or at least adding vinegar to it) and taking care of personal hygiene will be taught to all the populace. In the medical sphere, sterilisation of instruments will be implemented.
-- As far as inventions go, I'll implement a system of monetary rewards and public honour (fame and recognition) to rewards scientific discoveries and technological innovations. I will not, under any circumstance, implement a concept of intellectual property. In fact, it will be explicitly put in the legal code that no person can ever claim ownership of an idea.
-- Extensive infrastructural works, involving primarily road networks and irrigation systems. Using those roads, an imperial postal service (including an army-run high-speed 'pony express' using fast horses and way-stations where riders can switch horses) will be set up.
-- Given a blast furnace and, shortly thereafter, a heavy plough, the possibility to expand into more northern lands becomes interesting. Combined with practices of crop rotation (to be taught in all agricultural schools), we get some real perspective. (On the note of crop rotatation: the 'paper revolution' I'd intigate should be based on hemp paper, which is superior, and because hemp is a great rotation crop. It revitalises the soil. Also prevents some deforestation, and thus some erosion/desertification.)
-- A tree-planting programme to prevent OTL erosion and desertification in (among other places) the Levant due to the large-scale cutting down of trees to make ships.
-- Abolition of all slavery. Criminals and debtors can still be forced into indentured servitude to repay what they owe (to their victims or creditors), but there should be regulations and oversight to prevent abuse.
-- Citizenship depends not on ethnicity or religion, but on loyalty to the empire. All who serve a tour of duty in the army or navy get citizenship. Those who don't undertake such a tour of duty will forever remain non-citizen residents. As a result, they cannot own land (only rent it from citizens).
-- A legal system based on something similar to the old Germanic Thing is to be set up. That is: legal matters are solved by popular assemblies/juries. Only citizens get to vote in those assemblies, and all citizens get an equal vote, regardless of their station in life. All are equal before the law.
(Quick note: the whole distinction between citizens and non-citizen residents, as well as the prerequisite of military service to gain citizenship, is rooted in my belief that only a vast army can keep an empire of this size safe, and that proud citizen-soldiers are the most motivated soldiers you can have.)
-- The aristocracy is to become an aristocracy of merit. Non-aristocrats who behave in commendable ways get elevated to the aristocracy, whereas aristocrats who shirk their duties can be stripped of some or all titles.
-- All religions are tolerated in principle. The one exception is that if any religion tries to suppress other religions, all it's adherents will be put to the sword at once. This is very harsh, but history has taught us this terrible lesson too often to ignore it: zealotry cannot be indulged.
-- A system of succession to be set up. Ideally, I'd like to see the Emperor elected by a council of all the Satraps. I like elective monarchies.
-- A Constitution should be written to outline all the above.
-- Having done all this, I'd finally prepare the Empire to conquer Arabia and the western Med, just as Alexander himself had planned. The next stage would be to move gradually into Northern Europe, using my (relatively) advanced agricultural techniques to cultivate newly-annexed land bit by bit. I wouldn't be able to carry out my full plan within my life-time, but I'd set it up so that the empire's mission to annex all the inhabited world becomes almost an article of faith (or at least a deeply ingrained point of policy). Not because I like conquest, but because I'm pretty sure that this empire (which is multi-cultural and tolerant) could improve life for all the people within its borders. In any case, once Northern Europe is annexed, the population explosion there (due to heavy plough-facilitated agriculture) will make the Empire virtually unassailable.
-- Finally, I'd draw up some rather accurate world maps, and inform my political elite of the existence of the Americas. I'd also explain disease and inoculation (I would do that anyway), and specifically make it understood that any contact between the Old World and the New will lead to mass death. Since I'll hopefully succeed in making the Empire idenyify itself with a global humanitarian mission, I'll try to get the Empire to minimise the damage of trans-Altantic contact by providing mass inoculation to the Native Amerians. (This is more realistic than trying to prevent contact forever, which would never work.)
...To put it briefly, I'd try to make the old story of Alexander the Great wanting to forge a progressive, peaceful "Brotherhood of Mankind" into a reality.