With the king recently making his concubine the official second Queen there is a chance of him having a son with her (she is in her thirties).
In that case if no heir is proclaimed by the king that son would be the most likely candidate.
Otherwise the youngest son and the oldest daughter are likely candidates. The younger son is the only official son as the king has disinherited his older sons and by doing so their lines.
His oldest daughter is the only child born from his first marriage to a member of the extended royal family so could be the choice to prevent influence from her half-brother's maternal side of the family.