1 centurion: ubisoft has confirmed that if the community likes the rework of the shugoki there would be something like the centurion, and it is more than time to give rework to it, it at high levels does not work besides having no good openings and combos.
2 celtic: Celtic is a character you will never find in 4v4 matches, for the reason that he does not do well holding and doing gank, in addition to his light attack of 600 ms, he does not have good openings.
3 gladiator:
the gladiator should not undergo such intense changes, what more he needs is a better guard, nothing more.
4 orochi: besides to the community complain about the very fast lights of him, he still does not have good openings, he does not even have an unblochable attack, the only thing that makes orochi a viable hero are the deflects that you need to train a lot to get them to do.
5 warlord: he needs an unblochable to do in full guard posture
6 aramusha: his attack are very easy to predict and he has no opening
7 peacekeeper: pk has no opening, and also does not have much damage, it needs a little rework by adding an opening and increasing damage
I'm sorry if I messed up some things in English, I'm still learning
Highlander should be far behind the list, along with orochi, both are middlestrenght heroes. Gladiator needs changes, but I doubt he will get them soon. Cent, shin, pk and aramusha need it the most, the other 3 you mentioned should be the next in the line with kensei and warlord.
Cent needs one, Glad needs one, PK needs one, Warlord needs one, Orochi needs one (less light spam, more options for openers in Orochi's case).
I'd say a good 75% of the cast needs to be brought up to Raider's level.
I have HL rep 25, so I've played him a lot, and I don't think he needs a complete rework, he just needs faster light attacks, and maybe a 100-200ms increase on HA initiation for defensive heavy attacks.
The devs really should just give a blanket buff to all characters light attack speeds to 500-400 ms if they don't have them already.
Imo the following need a rework (in no particular order):
1) Cent
2) Shinobi (saw Freeze's vid)
3) Nobushi, devs were looking to nerf HS so they should compensate by giving her some QOL buffs (and maybe some new animations, to be less 'obvious')
4) PK
5) Glad
6) Orochi, doesn't have to be much, an ez to access unblockable would be neat
EDIT:
7) Warlord (heard Hormly scream in my head to buff his all block)
8) Musha, oops I forgot
Aramusha definitely needs a rework
His moves can easily be backdodged
In team fights he can easily be externally blocked
He has no opener other than his unblockable zone
To start deadly feints you have to land a heavy which is impossible at high level play since everything you throw is instantly parried
I think the Orochi, at the very least, needs to have an easier time incorporating his heavy attacks in his combos. I know people always get made when others light spam with the Orochi, but it's not like there's much choice. The Orochi's heavy attacks are easily seen, blocked, and parried, which makes it difficult for players to want to use them in a match.
The Warlord could definitely use a few unblockable attacks. His move list is very short, and they're all very simple, even predictable. The least that could be done to make him stand a bit more evenly with other heroes is a couple of unblockable moves.