"Investment" and "Car" are two words that have emptied the wallets of many a punter. Ask anyone who bought a Ferrari in the 90's...
Chances are a new Defender will be very expensive in the US, and it will be targeted at the luxury car market, with more electromagic controls, power mod cons, etc. The primary buyers of new Defenders in the US will be the Chelsea tractor crowd (or in US terms, the Connecticut tractor crowd). If we're lucky, the rap stars will stick to Escalades.
I'd think the impact would be similar to when Mercedes began officially importing the Gelandewagen to the US. Prices for GWagens all ready in the US didn't fall off the table. US prices still are stronger than ROW prices, and among off-road enthusiasts there are still top dollar trades for good trucks. Grey market imports continue, and those trucks sell for enough to make the importers keep bringing them in. And an added bonus -- as the trophy wives and rap stars move on to the next driveway accessory, there are decent deals to be had on newer GWagens.
Prices of the NAS 110's are stupid high because of the very limited numbers, and they likely will lose some value if new Defenders start coming to the US. I consider myself lucky to know the owners of a couple of the originals that actually get used off road as intended, guys who don't spend any energy worrying about resale value.
All that said, I'd never buy any car as an investment. Cars are tools and toys -- use them, enjoy them, expect them to depreciate. If for some reason the value sticks, feel fortunate.