I'm not saying the road building should be removed, just that I do not find it very useful. And yeah, capturing a fully working city is very good, but a lot of buildings will be gone anyway, you can't ensure the AI builds buildings/doesn't dip heavily into -GPT and stops progressing scientificially and the ability is not good enough a civ that gets almost nothing from peace should wait 2-3 eras for it when Authority + Colosseum's Production can get the buildings AI didn't get up (even if it only has a Shrine) sooner rather than later, all the while killed units give you a citadel to steal land with and possibly a Golden Age. Rome's UA is a very strong ability, but it's very far from being the strongest in the game unless you manage to steal really good UBs but that's incredibly situational and enemy civ dependent. Even then I'd likely rate the likes of Harun's/Sejong's/Dido's ability far above that anyway as they're reliable and almost always either way better early or superior in the long run anyway because only a few of your cities will have the stolen Roman UB, even less if the AI for some reason doesn't construct it in some cities, like Acropolis which I have problems finding even when Alexander is in medieval.
I'm not certain about tactical roads. Considering they take time to be built while stealing GPT which can't really be afforded early game (excluding +3 Gold monopoly starts), I'd rather just position my units well before engaging with my offense force so they only need a minimal amount of time before they kill. Takes a few turns, but building the road would most likely take more time anyway and if Legions are in such a position they're building a road near your enemy, they could just be attacking the enemy army instead. Sure, there's times I admit it'd be useful to do, but it's a rather situational thing that also takes time which sort of defeats the point. I did use them with workers, but that was IIRC before city connections and markets (or early gold in general) got nerfed to what they are now. I can't afford them and since their construction also costs me precious turns, they're not worth it very often. Ocassionally I will make one or two, but that's a rare occurence for me to find that worthwhile.
I don't really like the +15XP idea, a free promotion would be better because it wouldn't fall off as hard. 15XP is just a mere three hits for a melee unit and is more boring than a promotion because of how easy it is to get such small amount of XP for melee, whereas a free Drill promotion doesn't steal a level up, is better in the long run (and most often early on too) and opens up more possibilities.