One really obvious option I'd want if ordering a custom camper - I'd want the Long wheelbase van as the donor vehicle. I've never understood why VW don't offer it. We love our 2007 rusty roof 4motion 140,000 mile "veteran" Cali , but even for two it can be a bit cramped. 30cm longer would just make it a bit less squeezy and scarcely any different for manoeuvrability etc. Might even have room for three burners on the cooker!
Welcome to the club anyhow. You will find the Cali is better designed / resolved than any of the alternatives I think, although there are still plenty of areas that could be improved - but remain unchanged in three generations of the vehicle - VW tend to focus on changing the engines, dashboard, upholstry and styling (I'd stuff that changes for all the range anyhow) and leave the interior arrangement (stuff that is Cali specific) largely unaltered.