Summary
After some poor stints in Phoenix, Washington, and Sacramento, Ariza looked like he might be done being a legitimate rotational player on a good team. However, after being traded to Portland in 2020 he started playing much better, looking more like the two-way wing that helped the Rockets compete at the highest level against the Warriors at their peak. However, after sitting out the Orlando bubble, Ariza was traded several teams in the offseason and ended up with the Heat mid-way through the season.

Ariza has been solid for the Heat, giving them a versatile option at the power forward position. He's not the athlete he was in his prime, but he's still quick enough to defend on the perimeter. As long as he continues to hit shots at a decent rate on offense, he can provide decent two-way play. Ariza will be 36 as a free agent this summer, so I wouldn't expect any deals to be worth more than two years, if not one.
 
Cap Considerations
Ariza will be an unrestricted free agent with a $16.6 million cap hold and Early Bird rights. Using his Early Bird rights, the most the Heat could offer him is a starting salary of $22.4 million, which would be more than enough to retain him.

Since the 2021 free agent class is now looking less strong, there's a better chance the Heat decide to remain over the cap and bring back most of their team, rather than use cap space. However, with Bam Adebayo's max extension kicking in and Duncan Robinson set to get a big raise, the luxury tax could become a concern for Miami. If the Heat re-sign Ariza, it could cost them a lot in luxury tax penalties depending on what happens with their other free agents.

Other teams will have multiple ways to sign Ariza as even teams without cap space could possibly use any of the Non-Taxpayer, Taxpayer, or possibly even the Room Mid-Level Exceptions to sign him.