If I had to guess I'd say Sinckler would probably be one if/when he re-signs, if not already. TH props of his quality and standing are a rare commodity - and usually carry a pretty serious price tag (although he's only really blossomed into the world class prop we all thought he could be in the last 6-12 months). In terms of planning for his next contract though, I'd imagine that's a consideration to keep in mind when signing other players now - i.e. they'll probably have to make one of the marquee player slots available, if he's not already occupying one, when his contract comes up for renewal if they want to keep him (as I'm absolutely sure they do!) Goes to show how far ahead Billy and Gussy have to look when thinking about recruitment.
Other than that, no idea. Currently, maybe one or more of DC, Brownie, Robbo, Horwill? The issue is you have to think of it in terms of those players' value when they last signed their contracts. You could argue those guys have all been on the decline in terms of their perceived value within the pro rugby landscape since they last signed - but their contracts are still running so they're still on whatever was negotiated however many years ago they each signed. Danny's re-signing will probably have been negotiated at a lower rate than his previous contract now that he's not a nailed-on international and is into his 30s so I wouldn't expect him to be one of the marquees next season...
I don't think any of the clubs announce who their marquees are but it's often easier to guess - so Nick Evans was surely one of ours, just as Fekitoa will be one of Wasps' for example...