It's possible that the early death of Roger Mortimer, Earl of March, in 1398 was a factor in the Glyn Dwr revolt, because the Welsh had had high hopes of Roger. He became the first English earl to have an ode addressed to him by a Welsh bard, Iolo Goch, who was subsequently Owain's household bard.
From what little is known about Earl Roger, it appears he was both generous and popular. Indeed it was alleged that the reception he received from the common people at the time of the Shrewsbury Parliament was a factor in turning Richard II against him. Whether this is so or not, it is a fact that soon after Richard recalled him from Ireland, substituting Thomas Holland, Duke of Surrey as Lieutenant of Ireland. Mortimer was killed (in somewhat mysterious circumstances) before this decision could be put into effect.
From the point of view of the Welsh, the most significant fact about Mortimer (apart from the trivial detail that he owned around a third of the country!) was his descent from Llywelyn the Great (Llywelyn Fawr) through Llywelyn's daughter, Gwladus Ddu. In the 1390s it was quite reasonable for the Welsh to look forward to the possible crowning of a King of England with this lineage. (As an aside Adam of Usk claimed that Gwladus's mother was Joan, daughter of King John. This has sometimes been questioned, but it seems to me that Usk is as good a source as any other.)
The death of March and the accession of Bolingbroke killed this dream stone dead. However, as many of you will be aware, the descent from Gwladus was eventually transferred to the House of York in the person of Richard, the third Duke, through his mother Anne Mortimer. This blood descent from the House of Gwynedd was not overlooked in early Yorkist propaganda and was used in an attempt to attach Welsh support to Edward IV, with somewhat mixed success. This is rather ironic in the light of later history, and the Tudors making so much of their Welsh origins, which were, when properly examined, rather less impressive than blood succession to the great Llywelyn.
As far as I am aware Richard III made no attempt to highlight his Welsh ancestry, and in that regard at least he may have missed a trick.