Theis' contract is not eligible for extension.
jambr380's post properly captures our only real options with regard to Theis.
He's going to be very difficult for us to retain after next year unless something dramatic were to change with our roster that keeps us from being tax payers. And even if we had the non-Tax Payer MLE, that may not be enough to meet his market price, should he continue to develop his value the way he has so far.
For the Cs to stay under the tax, Danny would need something like the following path:
1) Spend only minimally this coming summer. Stash the draft pick overseas. Maybe sign one of Monroe or Baynes via the MLE. Let Smart and Larkin walk. Replace with minimum contract players.
2) The following summer, when Kyrie is due for a new, max deal, get Horford to opt-out and re-sign for a 2 or 3-year deal at a corresponding lower annual rate. That pays for Kyrie's raise without raising the team salary.