Sun roof.One problem intrinsic with hides is that you become slightly divorced from the outside - if a bird flies over you have no chance! Not sure what the (sensible) solution to that is.
Seriously, I remember twitching the Zitting Fan-tailed Cisticola Warbler at Sandwich Bay, and being very impressed with a hide there made from a shipping container. Looked pretty vandal-proof, so doubtless long since burnt out. Heated seats would be nice, if done properly then they should just warm backsides, not the air
For sea-watching, in the best possible conditions a conventional hide is not much help, because the rain should be driving straight in. Some sort of large open fronted barn like structure where you could sit well back from the weather might be better.
The hide on the Eden Estuary (at Guardbridge?) has heating and possiby even carpet from memory