Mortimer was considered heir presumptive to Richard.
"Richard II identified Roger as the heir to the throne, apparently in an attempt to thwart the claims of Henry of Lancaster (the future Henry IV). In so doing Richard was ignoring both the accepted rules of succession and the entailment of the throne drawn up by Edward III*. It filled the Mortimers with false hope. Richard never officially recognised the Mortimers as having a claim; in fact, he placed them in an inferior position to his uncles in the order of precedence, and pointedly refused to acknowledge Roger as his heir in 1394. Nevertheless, it was widely believed that Roger would be the next king if Richard died childless."