Since we are talking theories I have a crazy one, most likely it isn't true because it would make the story way too convoluted, but here it goes. I look at the art for Ugin and Nicol Bolas and there are some similarities between the two dragons and some very stark contrasts.

For one Ugin and Nicol both have similar horns and similar body types with long tails and slim legs and arms, their faces while different have mostly the same shape along with the rest of their bodies. Now this may be due to the fact that they are the same breed of dragon. But what if it's something different. Something more uncanny.

I've heard people say that maybe Ugin is colorless because he transcended color like the Eldrazi, I hope I have that right. His wings have feathers and the rest of his body kind of glows with raw power. Which is in contrast to Nicol who has scales and flesh wings like most other dragons.

So now that I set all that up, hopefully in a manner that is correct, my theory is that Nicol Bolas and Ugin are one in the same and that Ugin is the older redeemed version of Bolas who has transcended color and has traveled back in time to right the mistakes he had made as Nicol Bolas in the past. This would fit with the time travel story that is going on right now and would explain how Sarkhan was able to travel back in time.

Maybe Bolas did succeed in his grand plan and got his premending power back. Maybe his power is tenfold what it was before the mending but in the process he ripped the multiverse apart, and he is now trying to fix his mistake. Maybe this is true or maybe I am talking out of my ass, it's the latter for sure. However as silly as this theory is, writing it out has been very cathartic and if you read the whole thing sorry for my Grammer or lack there of.

Tl;dr: Nicol Bolas and Ugin are the same dragon just from different times and Ugin traveled back in time to try and fix his mistake of trying to ubtain his premending power.

P.s. Even Urza one of the most powerful Planeswalkers could not travel back in time more than a day or two and even then he could not do it himself he had to send Karn made of silver through time, but Sarkhan travels back millenia and he is no where near Urza's strength he had to have had help from something very powerful. Also time travel could still be very dangerous and could have sapped a lot of Ugin's powers to bring him more down to Nicol Bolas' level.