Working from your list.
1X Farseer (metal) Go in any list.
2X Avatar of khaine (Both Metal) Have his uses but easy to out manouver.
30X Guardians (15XNew Plastic, 11x old all same stance, 4 metal) They need some Heavy weapons, add the crew and grav platforms here. 3x 12 Guardians with brightlance give you much needed anti-tank, EML for all commers or scatter laser for anti-horde.
16xRangers..scouts..(Metal) Have their uses in most list.
6xWarlocks(Metal) Join with the Guardians for buffs and more WC-Dices.
8XSwooping hawks(Metal) Best used in a squad of 5+exarch to give them a large pieplate to throw around each turn.
2xSwooping Hawk exach (metal) Se above
6XDire Avengers(Metal) Cheep Little unit that can take a Wave Serpent as transport.
10xstriking scorpions(metal) Good against hordes.
7xwarpspiders(metal) You could need one more to have to units of 5, exarch inclueded that can jump around hitting rear armour.
6x Grav cannon crew and 2 grav cannon platforms(still metal) If these are the smaller platforms use them with the Guardians as above. If not, have them on their own. D-blast is not to be sniffed at.
5x Wriath guard(metal ... is that one lead?) Keepers, hard hitting and can take the hitting back. Joined by Fuegan you have a strong unit that can blow up anything in shooting and Fuegan can take the heat in melee.
1xHarlequin death jester (wax.. just kidding its metal)
1xfuegan the burning lance(ahh god more metal) Se above
1xeldar reaper missing his gun(Why metal there is so much)
1x wraith lord(made from super practical Metal) Give him a Heavy weapon of some sort.
1xWarwalker(God damn metal again) Two slots of Heavy weapons.
3xGrav Tanks (1 i gave to grim and is now a dakka jet) What types?
20xJet bikes(i think i dont know there are so many parts) best used in small squads for last turn objective grabbing.
3/4x vipers(again there are just parts everywhere) keep them cheep and give them range.
Without knowing what weapon loadouts the hevier units have there is difficult to give any firm advice. Their tanks is realy good and you could have on squad of 20 Guardians with 2 grav platforms and then have a unit of 12 or 11+ warlock in a wave serpent as DT. Then, like the DA you have two tanks on the field with massive fire Power and they can disembark onto objectives late game. Equiped right I Think you have most what you need.
Look for ranged anti-hordes, a Farseer on jetbike is better than on foot. He can go faster where his Powers are needed.