I reckon that whatever and wherever the new Defender will be built, it won't be a Defender anymore, however they name it ... I've no doubt it'll be robust, work well off road but I really doubt it'll be the box of Meccano we all know and love/despise, but to go backwards to old design engines is a big no-no in my book.
Yes they were and still are good engines but so much more could be done with a whole new design. I'd rather see a wholly new non-turbo design, for instance, with 'instant' power/torque that's lighter, more powerful and probably more economical and reliable than previous designs. If it's mated to a lighter stronger bodyshell it might be useful ... leave the suspension 'tuneable' and even a monocoque design might work ... or even spaceframe with a similar square body shape that can be changed from van to truck to softtop etc might also work well.
I see little value in keeping Defender going in it's current design though, for either customers or JLR ...
A contrary argument may be that there's little cost in tooling required to continue the present design, if it can get round environmental issues, it's an iconic design that is immediately recognised and known to have faults which can all be fixed or have been fixed at source. If only it would sell in large enough numbers ...