But I would suggest having it more as a movable terrain piece instead of a vehicle- I want to see characters jumping onto its back from high hab-blocks, and fighting atop the carapace and weapons!
It largely is designed to be terrain. And as Kaled says, there's a plan for scaffolding. So a lot of scenarios will be based around the "Stop evil person from starting/destroying the titan".
However, I fully intend to do the full-bore "God Machine goes to War" scenario at some point. It's a bloody titan, how can I NOT?
Duelling on its back... while it sounds cool, my worry is the same as Ynek's. It wouldn't be such a problem with a Lucius pattern, but the Mars doesn't have the same flat carapace. It's all slopes and curves.
However, that has given me an idea. If I've got enough clear plasticard, it should be possible to build a frame that could be fitted on top of the carapace to provide flat surfaces to stand on and perhaps a "safety barrier" of sorts around the edge.
Wouldn't look brilliant in photos, but it would allow models to be up there in relative safety.
Mind you, getting up there in the first place would be a challenge, as it's considerably taller than any of the terrain I can recall seeing at WHW.
Also, on a minor rules note, what effect will machine empathy have (if any)?
Titans have immensely powerful machine spirits, and Warhounds are even more raw and untempered than their larger relatives. Princeps are recruited from "amongst the Cult Mechanicus or even beyond it" (4th Ed Rulebook pg121) in order to find people strong enough of will to survive the mental link.
I pity any psyker who thinks they can control that - or the mind of a person who can.
Should I assume that the next edition of the Revised Armoury will include titan weapons?
There might be a teensy problem with the Encumbrance penalties:
As far as a Princeps, it is going to have a fully modelled interior and crew - who should be removable, allowing the option for a separately modelled Princeps and the possibility of an AdMech warband.