How about a Monk? Throwback to D1 Hellfire, and the only good character in D3 huehue
I think a Monk would be sick though, and to someone above, that does magic attacks.
The curse could change depending from the difficulty.
They could use the assassins skill sets. But how would you connect them to hell?
I like the idea of a new act 4 merc, something from hell would be cool like a demon or skeleton.
It would have made sense for act V (using the assassin as base), but some of the devs had a very strong anti-hellfire sentiment, so I guess it was off the table from the get-go.
For me, most logical thing would be an warden angels with magic abilities who are there to help us defeat Diablo, these angels have obviously followed tyrael and hadriel who are there to help us as well, in addition to the fact that historically it is a border between the high heavens and the burning hells, so the presence of warden angels would be more logical.
ps: it would be great to add some quests to complete the act4 lore about the past of those lands and see more in depth that act which is magnificent.