From the looks of things DE are getting updated very soon so obviously wait to see what that brings (I expect it will make them uch easier and much more viable to use).
I myself don't play DE but i will just give you general tips on what I feel needs to be in any competitive list, and I expect other people will comment proving me completely wrong but thats the beauty of a discussion
With the advent of 7th it makes it easier to cut out the weaker units that are filling the troops slot (especially in DE) though objective secured is incredibly good and DE's ability to get a lot of transports around quickly can really benefit them in Maelstrom of War missions. So try an weigh up the bonuses of that and if you can find an alternative go unbound and trying and push an armies strengths (though for unbounds to work you really need to be able to exploit it with at least 3+ of a power unit)
Objectives are the key to winning any game so you need to think how you are going to get them, whether its TaCos in whcih case you need to be able to move and adapt quickly, or standard missions and just be able to capture back fireld objectives while defending your own.
Then there is countering your opponent. Always make sure you have enough weapons to be able to deal with enemy vehicles. In 7th you can't survive by just taking a couple of higher str guns, you need specific low AP high strength guns so that you can get rid of vehicles quickly (lances will do the trick so make sure you have several). These should also be good for taking on monsters too so that I generally see as going into the same category.
If your enemy is running a horde army like orks, will you have enough sots to keep them back? Or get out there way? Or even be able to take them on in CC? How are you going to deal with large infantry blobs? lots of shots will always help but sometimes this means having to cut down elsewhere, though it does generally mean you get in more bodies which is never a bad thing.
DO you have any AA? now flyers are a lot less common nowadays but if your enemy brings one what are you going to do? Have your own is generally the best advice and as far as I'm aware theDE one is good (and there is meantto be som new bomber model for the new book).
Quite simply think about what the enemy is going to be bringing and see if you can counter it wether that is lots of 2+ saves, lots of bodies, vehicles high toughness etc. while trying to balance your own objective skills. A lot of the time you will have to skimp on some areas but as you play more games you will realse what is good or bad about what you have used.
hope that helped