To repeat what I said elsewhere:
"I have no beef with Tennant and Tate. My main concern is that Gatwa - and whoever his new companion(s) will be- deserve a chance to establish themselves without being overshadowed by old fan favorites returning. I note that RTD very carefully (and coyly) doesn't outright say that it's going to be a "multi-Doctor" story. I wouldn't be astonished if there's a swerve there."
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From what I've heard, Doctor # 14's debut is supposed to also serve as the series' 60th anniversary episode and be part of the BBC's 100th anniversary celebration, counting from when they first started radio broadcasts.
My guess would be that the guest stars help the new Doctor get his footing post regeneration and then 14 starts solo adventuring and gains companion(s) with the next episode, which will probably be counted as the first installment of his first season.
Besides, 13 is supposed to finale with a showdown against the Master and hopefully explain the whole "Timeless Child" scenario too, so 14 might be starting off with more post regen trauma than first suspected, similar to Peter Davison's beginning in Castrovalva.