Mmm marmalade! Nice idea to go for strong distinctive flavours.

I like it best with really chunky shreds of peel and with a dark brownish amber colour from brown sugar or treacle. Preferably quite bitter too.

I also like grapefruit marmalade, though that's obviously not seasonal in the way Seville oranges are.

I've had chilli laced apple jelly, and I think that would work in marmalade if done gently.
Ginger would be great in marmalade (I love preserved ginger).
Dried cranberries would add a lovely colour splash.