Future Trunks definitely would have been disqualified since he’s from an alternate universe and would be present in our universe as a result of breaking the taboo of the gods to time travel.
But it would be cool.
As for Cell, if 4 months was enough to get Frieza up to Super Saiyan Blue level, then they could have thrown Cell in the hyperbolic time chamber to get him up to speed if they were really concerned about his power level not being up to snuff.
But they stated they were concerned about tricks and tactics as much as power level and Cell had that going for him what with possessing the fighting abilities, powers and skills of all the Z fighters and the cleverness to make good use of them. He also had the combination of regeneration and Zen-kai boosts, so he could potentially get more powerful during the fight.