It looked to me like joe is in Scotland (looked local to me actually!) working security (how the hell'd he get a job in security?
) working ferries/docks. If mark lumps him into the sea with no witnesses & an alibi from Nige joe will be dead & mark will get away with murder.
Yes now thinking not Tom, Michael (taxi drivers stepson) and Leo (cocky rope boy) seems too obvious, he'll have been up to no good (related to computer stuff?) but not the rapes.
Could STILL be:
Jim - shit attitude to women inc wife, 'I can have sex with whoever I want' often cited as an excuse by rapists? When of course it's not about sex.
Ed - I really hope not because racist idiots would love that, I think at heart he's a good man though shouldn't have belted Jim (he could have had a 'strong word')
Ian - fits in with the seasonal aspect, if he's been spying on Trish raping her could be revenge for her not staying celibate after booting him out. We don't know he's NOT connected to the other 2 victims, it being a small town likely they knew him, they could've spurned his advances?
Arthur (country house owner) - we know VERY little about him, plus good actor be a waste not to use him, remember throughout the 1st series Joe was a 'minor character' seemingly only in it as millers husband till quite near the end we knew little about him.
Clive - unlikely I know as lack of dna evidence ruled him out. But still a misogynistic twat. Though I don't see his wife as a victim, more a case of they deserve each other, he cheats she's controlling (and lied that Michael was his?)
Caterer - as for Arthur very minor character becoming central would fit with broadchurch's style. Also fits with seasonal aspect
Band members - who are they? Could they be characters we've already met? Eg Leo?
Yea never liked Kath (though I like the actress) her 'I'm her best friend' Schtick seemed more about involving herself in the drama and getting some attention for herself than actually supporting Trish. As for 'why would he rape you' was a vile thing to say regardless of what she'd just been told.
I agree Chloe's situation will be why hardy leaves Broadchurch and the series ends. Tom & Michael likely to be involved in this storyline and while Miller would not condone Tom's actions I can see her still supporting him as he's her son and hardy and Miller falling out as a result. 
Eds daughter - can see her ending up under threat from rapist, having all her prejudices unravel (not drunk, not 'weak', not somewhere risky, wearing demure clothes & no make up etc)
At first I thought how the hell is eds daughter still on the case but then given its such a small town maybe that's not practical, maybe a lot of the officers on the case have connections to various people of interest in the case.