I'll go for two players who came through the Everton youth ranks and played for Everton but never really fulfilled their full potential. They never really became consistent regular starters for us and when they left Everton their careers just slid downhill fairly quickly.
1. Alan Whittle - made his debut at age 17 years. Made the England squad at age of 22 years but never got a cap. Was with Everton for 5 years (1967-72) and made 74 appearances and scored 21 goals from midfield (equivalent to a CAM of today) but never became a consistent starter. Left to go to Crystal Palace, then Leyton Orient, some Team in Iran I think and finally Bournemouth.
2. Jimmy Husband - came through the Everton youth set up and was with Everton between 1964-73. Made 165 appearances and scored 44 goals, mainly as a winger. Again was never a consistent starter during his decade at Everton (was prone to injuries also). Then went to play for Luton and ended his career playing football in the USA for 3 different teams.
You always felt there was so much more that these players could have given if they fully shined because on their day they were truly talented and skillful players.