I think the Rapport site is a decent site for a hotel personally, and that or residential is probably most realistic. Right next to St.Davids and the CIA and close to other hotels.
As much as I like the thought of a big office going there I would imagine that, other than Admiral needing a second new office, most of the new offices will be confined to one of four sites - JR Smart's Capital Quarter, Rightacres Capital Square, Callaghan Square South and south of Cardiff Central. All of that is a HUGE amount of offices for Cardiff so I just can't see the Rapport site having a new office there. I'd love to see it though.
On top of all the above there's going to be a fair amount of secondary space becoming available in the next couple of years with Admiral vacting Capital Tower and any other offices being vacated by organisations relocating across town to one of the big new schemes (such as Oakleigh House which alone is 41,000sq foot). See below
http://www.walesonline.co.uk/business/commercial-property/cardiff-office-development-oakleigh-house-7631652Although it's less interesting than shiny new buildings I'm becoming more and more curious what's going to happen to buildings such as Oakleigh House, Capital Tower and Bradley Court (where Morgan Cole/Blake Morgan are vacating). Those three buildings alone all in the same part of town are going to be empty within 18 months.