I was in a similar situation to yourself, i'd maxed my deathwing and ravenwing companies as well as tacticals in excess of 6000pts, and i did not know what to do, my wife of all people suggested i should start a new army, i ended up picking Chaos, and playing death guard.
I used some
AOBR terminators i had spare from collecting my 100 terminators for the Deathwing over the years, i used my bitz box and some green stuff and bulked them up a bit and filled them out with heavy weapons i had spare. they turned out quite well, and i get compliments on them at almost every game i attend.
if you fancy yourself to learn some greenstuff fu, take some time and a test model and work with it. i would try using play dough etc or none set model putty until you get your skills up to where you feel confident to do your first model.
Have fun and best of luck.
As for the Daemon Prince and the Defiler, the Defiler, is a unique model in and of itself. i honestly don't use them, due to there being better options for the H.Support slot, plus i dislike the model, but i know quite a few people who make Nurgle esq defilers with the Soul grinder model, which i admit is full of win.
The Daemon Prince i have converted my fair share of them, but the best base model i can think of for a Daemon Prince with wings is the Bel'akor model
GW has for order. one of which I have and use
atm.