Summary
Diallo had a bit of a breakout year this season, increasing his scoring from 7 points per game in 19-'20 to over 11 this year. His usage went way up, and he had some big scoring games throughout the season. However, these numbers came while playing for two of the worst teams in the league this year, so it's hard to take too much away from them. To be fair, the Thunder were actually decent during Diallo's time there.

Diallo lives at the rim, as the majority of his field goal attempts come either at the rim or in the short midrange, according to Cleaning the Glass. However, he doesn't finish particularly well and hasn't developed much of an outside shot yet, thus leaving him below average in eFG% for his position. Although he shot over 33% from three this season, the volume is low enough that it's probably better to look at his career average of 30.5%, which would need to improve for Diallo to really be a threat from outside.

Diallo is still only 22 and the Pistons liked him enough to trade for him and give him real minutes in the back half of the season, so I could see them overpaying him a little bit in order to lock him in long term.
 
Cap Considerations
Diallo will be a restricted free agent with a $2.1 million cap hold and Qualifying Offer, and the Thunder will have his Full Bird rights, meaning they would have no restrictions on re-signing him. If the Thunder don't think Vanderbilt is worth retaining for $2.1 million or more, they may not even tender him the QO, which would make him an unrestricted free agent.

Other teams will have multiple ways to Diallo as even teams without cap space could possibly use either of the Non-Taxpayer MLE, Taxpayer MLE, or possibly even the Room MLE to sign him to an offer sheet or outright as an unrestricted free agent. There's also a chance that Diallo ends up settling for the minimum. However, Diallo being a restricted free agent would greatly lower his interest among teams, as they will worry that the only way to pry him away from Detroit would be to significantly overpay him.