With the idea of taking this army and expanding it to 1500 and 2000 here are my immediate thoughts;
For about 400 points you can get 3x Razorwing Jetfighter w. Dissies - this is very powerful, supports rule of 3, is basically a Ravager lite (less killy, but harder to kill), and can be floated in as part of an Air Wing detachment for a bonus CP.
To specifically hot 1500 and keep your original list untouched you could add in a squad of Scourges w. 4x Haywire Blasters (I personally prefer regular darklight Blasters, but am trying to work within the confines of adding 500 points to the existing army)
These two additions will give you a vastly increased amount of firepower (6x Dissies, 4 Haywire blasters, 3 missiles, and a decent handful of additional poison) will give you a deepstrike unit that can slap out mortal wounds, and will give you all the maneuverability and blocking potential of 3 large base flyers.
To bounce up to 2000, I personally lean getting yourself an assault element for counter charges and to harass gunline builds. I think this is the logical waya for DE to go as we do have some excellent assault tools compared to most gunline armies, and due to rule of 3 after we take 3 Ravagers and 3 Razorwings we actually start petering out on optimal gunline options (sure, you could pack in Reapers and more Scourges if you want, I just find the assault tools more useful - but if you hate assault there are some other ideas)
At that stage your best assault options are to go Cult or Coven - there are some pros and cons to each (and even some personal play preference).
I would tend to advocate the following rough build starting points;
Cult: I advise Red Grief as the choice, but that's because I aim for optimal Succubus - if you want to bring more Wyches maybe aim for Cult of Strife.
Succubus w. Blood Glaive artifact (this can get you a Succubus who can actually go toe-to-toe with a Knight and do some work, much less blendering infantry)
Get a Raider (w. Dissie) and a squad of Wyches to support her, bring a net.
The above can run you around only 200 points, and you can use the extra points to bring more Warriors w. Blasters (I use Raiders, but you can go Venom if you think that's better) in your Battalion.
You could also go with a second squad of Wyches, or maybe float in some Reaver Jetbikes which are awesome (and also benefit from the Succubus' aura).
Coven: I tend to advise Prophets of Flesh for the CP benefits, but you almost can't go wrong with any option - I think Scarlet Epicurean is great on Grots and Talos.
Take a Haem (with his Warlord trait via Alliance of Agony you can basically 66% of the time get extra CP and 100% of the time not lose anything for trying) I'd equip him for brawling with an electrocorrosive whip aka the better Agoniser.
Take a squad of Wracks in a Raider (this can be as cheap as around 125 and nets you a Dissie Raider and a squad to camp objectives)
Then take a pile of Grots (4 in a Dissie Raider can run you as cheap as 220, or you can footslog them0
But Grots are great fire magnets, decent in assault, and super resilient - so will absorb a lot of fire from your enemy. You could also opt for the elite detatchment for extra CP if you want to go with a lot of Grots (you'd need to trim other stuff, natch)
The Wyches are a scalpel - they're very fast, can cut deep, lock down stuff great with their nets, but are a touch fragile.
The Grots are a hammer, slower, clumsier, not as flashy, but durable as heck and not something that can be ignored.
Choose for your playstyle/budget/what you'd rather collect. Both options can be very lethal.
Those are my quick thoughts for expansion - hope they help!