I'm curious as to what things would physically look like had Rome not fallen? What clothes would look like, what buildings would look like, the design of the military uniform, how things as basic and varied as carts and wagons and containers would look, etc.
There are a lot of butterflies concerning fashion over 1500 years. Look at the history of the tie, e.g., and you realize that it is a far from inevitable item.
So, you can pretty much do most things you like - but in the long run, they have to make a little sense. For inspiration, maybe get a closer look at changes in late antiquity or in Byzantine culture, concerning the design of things. Maybe you can find a line you can continue there.
I'd imagine men would wear skirts forever - maybe pants would be "exotic" clothes. Military uniforms would look similar, but with skirts and camouflauge leggings. Medieval castles/architecture would look fairly similar - I think there would still be a "gothic" building style with flying buttresses and vaulted ceilings that develops, but it would probably be less ornate. I'm not sure if they would adopt wooden houses, but I suppose that would be an eventual development.
* a tunic is not a skirt, but a dress, technically. But I agree when it comes to medieval regions. When I travel there in summer, I often wish I could still dress like a Roman! In the Northern parts of the empire, even Romans adapted to leg-clothing OTL, as far as I know, and I guess this would develop into our sort of pants (took ages OTL as well!). Maybe in these parts (if they are still Roman in the timeline), traditional "Roman" clothing would only be worn on formal occasion.
* Same about wooden houses. Of course there were wooden houses in places where there is a lot of wood, and this would be the case in a Roman Northern Europe.
* medieval castles would not exist the way we know them. A surviving Roman empire would probably still rely on a sort of professional army instead of a cadre of feudal elite soldiers based in castles. Roman fortresses would rather directly develop from antique form into the early modern "Vaubanesque"-forms
* Gothic building style is a bit like trousers. You can of course let it happen, but I do not deem it probable. In OTL, the most prominent examples of Goth architecture are Northern European; it didn't get very influential in OTL-italy even (that's were the term comes from, Italians called it Gothic meaning Barbaric). Maybe we see rather an emphasis in domes, meaning that churchbuilding develop directly from the Hagia Sophia to Rome's St. Peter.