Welcome to the forum!
Death Guard is fun indeed, especially regarding conversion possibilities. Furthermore, Death Guard will be included in the new edition
40K box, so you would get some units together with the rules (and you could sell the Space Marines). You can check my Death Guard in my gallery for inspiration.
Orks are fun and there are a lot of 3rd party manufacturers of Ork figures (check out Kromlech (Bits of War)), should you dislike
GW's ones. I made a Grots (counts as Orks) army and you can also saee that in the gallery or following the link.
I never liked Tau, or T'au as they will be called soon.
Tl;Dr I would go for death Guard as they come in the starter box and they are not Orks which you painted for a long time