If you aren't satisfied with the current state of sieges in Total War: Warhammer and really want to make something special, you're in luck as the official Battle Map Editor is slated to arrive later this month. Once it comes it will bring with it pretty much everything you need to create a proper Total War: Warhammer map, everything from props and environment models, to SFX and VFX. But most importantly of all, it will come with an easy one-click export and Steam Workshop support, so you'll be able to easily share and download a whole variety of maps.
The catch is that the maps will only work in custom and multiplayer battles, so if you were hoping to adjust some of the campaign maps I'm afraid you're out of luck. On the positive side, the modding scene for Total War: Warhammer has proven to be surprisingly crafty, so who knows, maybe they'll figure out a way to import these maps into the campaign!
That aside, if you're wondering what Battle Map Editor looks like in action, here's the recently posted preview video that covers almost every aspect of it:
The full release is slated for later this month, but if you would like to help the developers test Terry (and yes, the Battle Map Editor is called Terry) you can find all of the information you need over at the official website. This version of Terry is only available to Windows 10 users, but don't worry, the full release will be available for both Windows 7 and Windows 8 users as well. As for the recently released Linux version, I wouldn't hold my hopes up given that a completely different company handled the port. On the positive side, the maps are most likely going to be compatible, so you'll still get to enjoy the various penis-shaped maps the community will inevitably come up with!