Don’t think the DART being made airside would be the worst idea ever - if you made it hand luggage only, or at least segregate trains to either being checked luggage / cleared security. It would probably give LTN another USP. There is certainly the space to put a number of conveniences and about 4-6 security lanes.
Alas the land has already been earmarked for other projects I believe.
At LHR, on the 2 days after the closing ceremony of the Olympics in 2012, they opened their “Games Terminal” which was in essence the same remote terminal philosophy.