Does anyone know if it's possible to make a user created campaign and have heroes carryover (with their items) to the next map? It was possible in the official campaign but I couldn't see any options to do it in the map editor. Edit: This relates to AOW: SM.

Sadly not possible (except perhaps if hacking files) in AOW2 or AOW:SM, only in AOW1.
If you want player made campaigns, I remember Artic Wolf made some excellent ones for AOW1.

Oh well, a pity. Thanks for the reply Petrus, I'll have a look at the player made campaigns you suggested.