Trell has been a character of interest in the Cosmere ever since the Alloy of Law book. There are tons of crazy theories on Trell and Trelagism, so I thought I'd share some of mine.
Trelagism reminds me a LOT of the Shardworld Taldain. One one side it's constantly daytime - Dayside - and on the other it's constantly night - Darkside. In the religion Trelagism, worshipped by the Nelazan on Scadrial, they thought that night was sacred, as they could see the Thousand Eyes of Trell, which were the stars. This could be a reference to Darkside, and it's entirely possible some Darksiders travelled to Scadrial and spread their beliefs.
White Sand takes place waaay before Mistborn. Trell, supposedly an Avatar of Autonomy, could have gone to Scadrial himself.
In Trelagism, Trell was the stars and Nalt was the sun. This isn't something crazy, but I connected Nalt to Nalthis for some reason. Is this a coincidence or am I connecting two random things for no reason.