There is the possibility of plastic super heavies being constructed. Gw are trying to do a baneblade, and demon engines. I had heard that the stompa in plastic was abandoned (for the ork codex at least) due to the difficulties and cost in casting it in plastic (as with the New avatar concept for eldar that was passed onto FW).
Supposedly it will be part of the 40k Codex Battalions. Where you can have huge Super heavy battles. Basicaly it will be a "canon" version of Imperial Armour that will be useable in GT and other such "offical" GW events.
But to be honest I don't see the point in making plastic Baneblades other than saying "look we can do it" cost wise it will most likely be a similar price to the forgeworld one anyway. I mean come on a Land raider is 30 GBP. And even normal transports are 20 GBP now.
The resin one is only 140GBP, I'd expect GW to push this model in plastic at anything near the 80 - 100 mark. Or even more possibly.
Personally If I were going to buy a Super heavy it would have to be the Shadowsword. Its the only kit with a fully detailed interior. For me its much better value than the the others as they are just shells essentially. Thats going to be the same with any plastic kit GW does. No interior detail at all. If I'm going to spend that kind of money on a model I want it to be something with modelling and conversion prospects. Fully detialed interior with crew, hatches you can open to see inside, the prospect of adding your own elements in bits and plasticard etc.
Not just an overpriced Rhino.
On a side note I heard from a guy at FW that the Reaver Titan is due for a Christmas 2007 release. If they bothered to make a Manta I can't see why they wouldn't make a Reaver Titan.