While I don't think juggernauts compete too much with wardens specifically, as the latter has utility the former don't, I do agree that fighters in general have too much overlap with tanks, particularly when the recent Vanguard Update gave some of them very similar playstyles. With the exception of a few vanguards like Sejuani and Rammus, who I think are juggernaut and diver hybrids respectively, tanks should not be dealing high damage, and the fundamental difference between them should be that tanks should be using disruption as a means of attracting attention towards themselves, whereas fighters should be damage-dealers first and foremost, and damage absorbers second. I think separating the two is going to involve work mainly on tanks, rather than juggernauts, though I agree that the latter could also afford to have more unique tools at their disposal.
I also agree that juggernauts should have a baseline level of engage, which I think is also what makes Darius and Urgot successful: while engaging on the entire enemy team should be difficult for a juggernaut, I think it would be perfectly okay for them to have the tools to start a fight with a single opponent, since they have so much riding on their AoE that they wouldn't be at full power when dueling them (or shouldn't, at least, unless they're specifically duel-oriented like Nasus or Urgot). More generally, I think all melee classes as a whole should have tools to force fights, more so than ranged champions, because their range limitations means they need to be more proactive in entering and leaving combat.