Where Can Doctor Who Go After 'The Timeless Children?'
The Gateway Monument
When Tecteun landed on that fated planet all those millennia ago (and just how long is unknown to both us and The Doctor), she didn't immediately find the Timeless Child. The first thing she found was the monument.
A gigantic monument built by peoples unknown, this contained a portal to another world or, possibly, another universe. Tecteun herself looked through the portal, and it's implied that when she returned she found the child at the foot of that portal. Did something follow her back from its own world? The odds against something just happening to come through that portal while Tecteun was there, and having a similar physical appearance as well as internal organs, are astronomical in an infinite universe.
Could it be that Tecteun was more than an adopted mother to The Timeless Child? Could her entering that energy field have allowed something from another universe to merge with her base genetic code to create something that she'd find appealing? And if so, what is the purpose behind creating an unearthly cuckoo of sorts that would be taken back to her home planet, changing their entire evolutionary tract?