I'd like to know what could happen if I finish Morrowind's expansion Tribunal before finishing the main quest. I mean, will I get powerful items, or be able to talk to Vivec if I haven't met him yet? Can I kill the dummer king and take his ring? That sor of things
No, as far as this site explains, the extension assumes that the main quest is complete (or at least progressed far enough). Though it may confuse one if they never started the main quest, it should be ok to do the extension pack before the main quest.
EDIT: Killing the king won't have any negative effect other than losing a reward from him, according to this forum and a wiki excerpt one user included.