Which poses all sorts of interesting questions about an *Age of Colonization analogue....
As well as an *Industrial Revolution analogue.
For one, TTL surely will not see anything analogue to the Dark Ages. If you expand Roman socio-economic urban market model up to European Russia, and integrate all the barbarians there in it, you have removed the possibility of a permanent collapse into feudalism, the social and economic base for the Empire would be far too strong. The possibility remains, just like the Chinese, of recurrent dynastic collapses and breakups from civil wars, economic slumps, and big Hunnish invasions from Central Asia, at least as long as the Empire remains pre-industrial, but the Empire is sufficiently big and strong to recover and/or integrate any invaders. According to the Chinese model, any such collapse would not last any more than 1-2 Centuries.
Which means that, assuming the Super Roman Empire would still have the equivalent of the Third Century Crisis, from civil wars, and the equivalent of the Fifth Century Crisis from the Hun invasion, we can likely see to a recovery in the Sixth-Seventh Century (the equivalent of Justinian's reforms). Another crisis or two can be expected (e.g. from the Mongol invasions _if they haven't already discovered gunpowder_ and the Black Death).
Assuming that the continuation of the SRE does nothing to stop the technological progress OTL Europe had in the Middle Ages (hard to see why), and that it proceeds even more swiftly with the lack of the Dark Ages and better contacts with India and China, and the integration of Parthia and Germania-Sarmatia, the SRE could have the technology base to launch its own *Age of Exploration as soon as 800-900 CE. What it all takes is some Emperor willing to found it. Celts and Germans had legends about lands to the West... Anyway, circumnavigation of Africa, and building of sea routes to India and China will be a given, even if, with full Roman control of land routes up to the Indus, they will not be so vital.
Of course, Rome would need a political structure somewhat sturdier to keep control of the whole Europe-North Africa-Middle East landmass. The Tetrarch experiment was a recipe for recurring civil wars and breakups. Either they develop the equivalent of the Chinese bureaucratic elite (a definite possibility, Roman Cursus Honorum was not so different), or a federal senate with representation from cities and landowners all over the Empire. Slavery will be most likely more and more marginalized in the core Empire by the economic changes that the *Renaissance will bring, but it may see a nasty comeback when the Empire colonizes the Americas.
The burning question is whether the condition of Europe as a unitary super-state, instead of a Balkanized mess of city-states and absolutist nation-states, will do anything to hamper the transition to the Industrial Revolution. ITTL, the race is open between two comparable Empires, SRE and China, for the Industrial Revolution. The first one to achieve that will break the dynastic cycle and grasp the bases for a worldwide expansion.