I complete disagree with this sentiment. Boba Fett has, at best, been a side character in the Star Wars movie and TV shows. And as such, he should continue to do so. Having him front and centre with lots of screentime robs him of much of his mystique; which was what drew fans to him in the first place - the guy being a silent professional, competent and efficient at his task of capturing Solo, and having the stones to talk back to Vader without getting killed; all while remaining mostly in the background.
Having Boba Fett train a team of bounty hunters and/or Mandalorians would be confusing to the viewers thanks to their identical helmets; in which case, the main way to differentiate him from the others would be to have him without his helmet. Which is again, in my opinion a no-no.
Lastly, having multiples of him cheapens and, by necessity, weakens each individual - thanks to the Conservation of Ninjutsu
. One of the smartest things Lucas did was to make Boba Fett an individual. Originally, he had intended the bounty hunters to be an elite team of mercenaries in identical armour who Vader hires to hunt down the Millennium Falcon and its crew. However, he wisely decided to make them more individualised explicitly so that when one of them reappeared to trap them on Cloud City, the viewers would recognise said individual.