Finding any viable development at the moment is tough going indeed.
As a site, I'd suggest you look at the one in Douglas that covers the cinema and the petrol station. Douglas hasn't suffered the same level of business closures and empty premises as other areas of the city. And that site is very low density. Working against you is the fact that there has already been a planning application (refused) on the site, which would restrict the development possibilities. It would be best to incorporate a new cinema into the site,
In terms of other developments, a multi-sports centre with facilities for non-mainstream sports (i.e. not GAA, Rugby, Soccer and maybe Basketball) would be useful in the city. This would be especially true on the Northside. Also, there isn't a proper velodrome in the country (there is one sub-standard, outdoor one in Dublin). Incorporating an indoor one could have some benefit in funding as cycling is an Olympic Sport and could produce future revenues, as it can also be used as an indoor concert venue.