I am going through the main quest for the first time, and then I am going to play the expansions, also for the first time (I had the game years ago but only messed around). Anyway, I was planning on using the same character I beat the main quest with, but he is so strong now I think it may take the fun out of it. Some boss enemies and hordes of Daedra can still be challenging, but honestly the game is just me leaping from mountain to mountain with ease.

I am worried that I am too built on this character (level28 as of now, expert marksman, 100 acrobatic and 300 speed, plus good alchemist so constant potions) and will make the expansions less fun.

Do you recommend making a new character to play them? And what is a good level to start each expansion at if I do?

How much is your health? Why don't you just play Bloodmoon and Tribunal and see if you are truly powerful enough to take them on? I find the final quest in Bloodmoon hard even with a level 50 character with full attributes and full main skills (on middle difficulty). Those expansions are developed to challenge high level characters to begin with.

You could always stop using potions that much - it will increase the difficulty.