If Edward IV of England (born 1442, reigned 1461-70, 1471-83), head of the House of York, had not died young there would have been no Richard III, no 'Princes in the Tower' conspiracy industry, and probably no Tudors. Tall - over 6 feet - and physically strong, charismatic, a skilled warrior and general, and resembling his daughter's son Henry VIII, Edward also had a reputation as a womaniser and ruthless traits. Like Henry he seems to have been sporadically lazy and to have become overweight in middle age, and his controversial marriage to the 'Lancastrian' faction widow Elizabeth Woodville (5 years older than him and of gentry status not noble or royal, though with distant European royal blood) had already led to a split with his cousin and chief backer the Earl of Warwick ('the Kingmaker') who overthrew him in 1470. (This is the background to the TV series 'The White Queen' with EW as the said queen, but the stories of witchcraft etc in it from Philippa Gregory's books are unlikely and probably only propaganda by her enemies.)
Edward fled to the Netherlands but later fought his way back to power, aided by his youngest brother Richard of Gloucester (ie R III), killed off his Lancastrian rival Henry VI and the latter's son plus Warwick, and seemed set for a long reign. He had two sons and six daughters and no discernible rivals.. then he died suddenly aged nearly 41 in April 1483 and left his throne to a boy of 12, Edward V. Elizabeth Woodville's family and Richard feuded over the regency, Richard won, Edward V was deposed on an allegation of his parents' marriage being bigamous so he was a bastard, and R became King. Edward and his brother Richard of York, aged 10, were kept in the Tower of London for 'security' (probably due to genuine Woodville plots to free them and depose Richard not just as an excuse), and disappeared, and this wrecked R III's reputation among a large section of the Yorkist faction. After one failed revolt their mother Elizabeth W agreed to her eldest daughter Elizabeth of York marrying the last Lancastrian claimant, Henry VI's exiled nephew Henry Tudor. R III's only son then died aged c.10, destabilising his regime further. HT invaded in 1485 aided by France (which R was threatening), and the result was the Bosworth debacle and Henry on the throne marrying Elizabeth of York - but assorted pretenders claiming to be the missing Princes in the Tower.
Although Edward IV had apparently developed a weight problem in his 30s his death was a surprise, and poison was inevitably rumoured though it was put down to a fever/chill caught fishing on the Thames at Windsor. Richard III claimed he had been weakened by years of Woodville-led debauchery. Had he lived even a few years longer his son would not have needed a regent ('Protector'), and in this era kings could normally rule unaided from the age of around 16 - which in E V's case would have been in 1487/8. The prince was bold enough even at 12 to protest at his uncle Richard arresting his relatives in 1483 so he was evidently capable, and had he become king as an adult, with a brother to succeed him if he had no children, it would have been Richard who was sidelined and under suspicion of plotting. Richard's brother Duke George of Clarence had already been executed in 1478 for apparently trying to deny Edward IV's sons the succession, and in 1447 the young Henry VI's uncle Duke Humphrey of Gloucester had been arrested and died suddenly after a 'plot'. Would Richard have gone the same way (or been killed trying to resist arrest or had to flee the country) and been merely a footnote in history , and the Plantagenet line continued under Edward V?
In the case of no Tudors, logically there would have been no royal 'Break With Rome' in the 1530s and no Reformation , but a continued Catholic England - though possibly a cash-strapped king would have closed down some monasteries to 'reform the corrupt Church' and gain extra income. The Boleyns and Seymours would have been unlikely to gain any power unless a later Plantagenet king chose a domestic not a foreign wife, which was usually ruled out due to the fear of such a match causing inter-noble jealousy and plots from defeated rivals. And would Edward V have taken more interest in John Cabot and Christopher Columbus than the preoccupied, politically unstable Henry VII did, and sponsored a greater English commitment to early exploration of the Americas?