Lee Thomas (if he comes back and continues in the same vain as the start of the season), Andrew Bishop (if he keeps up his form), Roberts (should be replacing him anyway, but has been injured for a while), Gareth Owen (needs some experience first, but could be class in the future) and Jon Davies (if he ever meets his much lauded potential).
There are many candidates, but none as of yet are really fit to take Shanklin's spot, unless he plays **** for Wales in the 6N like he's done for 8 months.