Unrealistically: Wendell Carter Jr. I think he's mini Al with some "attack the closeout off the dribble" potential. I'd be ecstatic with John Wall if he declines his player option, but he won't.
Semi-realistically: I'd love to see Malcolm Brogan. TJ Warren or Gary Harris would also be great.
Realistically: Josh Hart, Nic Batum, Kuzma. Rubio off an injury is worth a flier, imo. So is Ingles, but in more of an "11th man" sort of way than an "assumed rotation player" like Rubio. PJ Tucker would be nice Grant mentor but wouldn't really move the needle.
I'd talk myself into it: Iguodala, Thad Young, Ibaka. Nice guys to have, but I don't see them actually moving the needle by becoming a top-8 rotation player. I'd take a Bagley one plus one if he's willing. Taurean Prince and Malik Beasley would be interesting, but also guys I wouldn't slot in as rotation players on paper.
I'm all for Beal if he wants to come here. Think he'd be happy to be 2nd or 3rd banana to mentee Tatum and young and hungry Jaylen.