I mean will we see the Doctor fill that tomb someday or has it already happened and time has been rewritten from some time?
There's no good answer really. Heck, even Steven Moffat doesn't have a good answer (this is supposedly The Doctor talking)
Changing time is tricky. It’s a bit like a detective story: so as long there isn’t an actual body, you’ve got a certain amount of wiggle room – for instance, if the body has, rather conveniently, been burned on a boat in Utah. Here’s the thing: I can change the future so long as the future has not already been established as part of my own past. I can’t rescue Amy and Rory because I already know that I didn’t.
But what do I know about Trenzalore? There’s a big monument that looks very like my TARDIS. There’s a temporal fissure leading to my timeline. Maybe it’s my grave. Maybe, one day, it’s my burial ground. Maybe it is something else entirely, and we got it all wrong. Don’t know. Don’t plan to find out for as long as possible. The main thing is, Clara still jumped into my time stream, and ended up helping me through all of my life. All that is established, unchanged – but there’s wiggle room!
So there you have it. Because
reasons timey wimey Steven Moffat
There's some problems making it a future grave (why didn't we see #12 in the scene with the Doctors running around Clara?) but nothing that can't be waved away. And if I were going to die, I'd pick the spot where everyone says I was supposed to die.
The way I see it, it is a future that probably won't happen anymore, like the twelfth Doctor explained in the episode In The Forest of the Night
CLARA: But we saw the future. Lots of futures. Earth's futures.
DOCTOR: They're about to be erased.
This implies that futures that are not fixed points in time, can be rewritten.
Originally, the Doctor's last incarnation (Matt Smith's Eleventh Doctor, who was actually the thirteenth and final) was supposed to die in Trenzalore, defending the town of Christmas. Thanks to Clara's intervention, the Timelords granted the Doctor a new Regeneration cycle, so the future we saw in The Name of the Doctor never actually happened.
(Which causes kind of a paradox, because if it didn't happen, then Clara didn't become the impossible girl, so she couldn't travel with the Doctor at all, so she couldn't convince the Timelords to help the Doctor, which would have caused his death in Trenzalore after all, so....my head hurts).