I know what you mean about the Bikers - there is a LOT of plasma around right now (the Plasma Syphon in the GK book doesn't look so stupid now) and my Bikes took a pasting there.
Yes, they are Plague Marines - I would actually likely use more of them rather than less, because they were utterly awesome against Wolves - no charge bonus for them meant they struggled to get through the Death Guard's foulest, and with a full Tally active, they butchered the attackers.
I'm actually selling my Helbrutes right now, they are so average now it hurts and I can think of better units to spend money on and look pretty. Havocs do look attractive, perhaps they will be required for anti-air. I am liking my Forgefiends backed by Warpsmith behind a defence line idea though.
I'm thinking Zombie Blobs too, to tie up enemy units. As the game progresses, they become tougher and tougher. Or even plain Mark of Nurgle Cultists - a full Tally gives them AP1 weapons!
I am fairly sure that GW WILL nerf the Epidemius ability, but I'm wondering how much. They have already FAQ'd the Daemon Codex, even recently, and nothing has been changed. I can see perhaps they will change things so FNP is improved to 4+ rather than 3+, and maybe AP3 rather than ignoring Armour entirely, but frankly it makes the Death Guard VERY fluffy, and besides the opponent needs to find ways of taking Epidemius out rather than nerfing his abilities to not allow the combo. Right now, I'm enjoying 40K a LOT, because it's others working out how to beat ME rather than the other way around for a change!