Official mod tools and Steam Workshop support for Total War: Attila have arrived. And no, there are no paid mods.
The Attila Assembly Kit (in beta) is the most extensive yet, and offers a trio of lad-named tools: DaVE (database editing); BOB (processing of raw data); TeD (creation of individual battlefields for land, sea and sieges); and Terry (campaign map editing).
In addition, there's a Terrain Raw Data resource pack that includes a gigantic 60GBs of in-game production data to mess around with.
Hit-up the Total War Wiki (and then ask the community) for any help.