Deathwatch kill team unit adjusted to work as a veteran marine squad. Should it have the heavy bolters normally used by deathwatch or switch to two heavy flamers to help with close range attacks. The bonus no rolling to hit with the flamers means it is simply down to wounding. It wastes their good ballistic skill but the quality of the weapon should make up for that.

A flamer marine could be converted to have what looks like a big bug spray cannister. Or design it so it looks like a big fuel tank on their back with a nozzle ready to spray toxin/poison over everyone. Get one of the imperial guard ones as a starting point for the conversion. Possibly convert the flamer so it is a bit shorter in length.

Magnetise a landraider so all options are available.

Add a second predator to help boost the armour of the army. Magnetise the weapons, so one of the landraiders can be upgraded to a terminus pattern with the single las cannon sponsons.

Add a dreadnought ironclad for some serious close support for my terminators. Needs to be pretty heavily converted to stand out from my normal dreads.

Build up a landspeeder squadron to help deplete enemy numbers and have a couple of extra shots in case of misses. Also great for use as a skyfall spearhead unit to destroy multiple enemy vehicles or infantry units.

Would a pair of whirlwinds add some decent cheap artillery options to the army? Or should it be a whirlwind and a vindicator? The whirlwinds could be converted to match more the manticore look rather than the current design. The back mounting carriage and the driver compartment could be used to fuse the manticore missiles to a rhino chassis, swap the guardsman driver for a marine and you have a pretty fancy conversion with big rockets for use on the tabletop.

Use the immolator as a basis for a razorback for a veteran / elite unit or for a command unit. It wouldn’t take much work to replace the gunner with a marine one and swap out the weapons for something within the marine list. Might be worth trying to find an original metal conversion kit rather than the new one.

Building an effective force would only really entail a few more vehicles for transport and a couple of special and heavy weapon options for the squads to bring them in line with the rules.

Objective markers. Do a set of objectives one through six looking like scanner blips on green and then replicate exactly but use red as a colour so switching the objective control is easy. Have a grey version for any uncontrolled or contested objectives?
Double sided ones! Red one side green the other. For the contested or unclaimed versions one side could be blank (grey) with the second side half red half green (in a yin yang style?) or should it be grey and blue sides to cover multiplayer games?

Captured banner marker, either have it as a simple little circle marker or possibly as a little flag on a base.

Destroyed vehicle markers.
Have a couple of junked rhinos and other vehicles to help block line of sight when they are damaged.

Build the grand master of the grey knights as either a terminator librarian with deathwatch experience or as an inquisitor. Either way it adds a powerful and expensive unit to the army. As a senior inquisitor, it would give access to the mystics and heavy weapon servitors to help defend the army firebase from drop armies and from highly psychic armies. The main focus would be to just get an awesome model onto the table. Rumours are that mystics are going to be dropped for inquisitors so I might hold off on the extra unit models until the rules are closer and we have some idea of what they can take.

Dreadnought – either venerable or standard. Can they still have the twin linked heavy bolters? If so, them paired with a close combat / flamer would be useful in close support against infantry and light vehicles. Is there the option to give twin linked autocannon on each arm mount? Good for heavy infantry and vehicles, and looks AWESOME! have the arm options magnetised for easy weapon testing AND leaves it open to be further upgraded if other options become available or if I want to run it as an ironclad for a game or two. The blood angels can get the redeemer flame cannon on their dreads. Might be worth building one up and run it either as an allied unit or use the weapon as a normal heavy flamer until there is a rules update. (or I pick up the Imperial Armour book to use it in my Apocalypse games).

Redo the unforgiven dark angel character as a chaplain for the army and give him matching bolt pistols or plasma pistols along with converting the model so it has a much stronger chaplain feel to it.

Strip and repaint one of my classic techmarines (beaky MK6 marine) and convert it so it matches the current rules (servo harness)

Possibly convert up a version of the tech marine light artillery piece for my army to use. I don’t see it in armies I play against so is it a useful addition or is it better to spend points on a whirlwind or vindicator?

An assault squad in a rhino is an easy support choice for countering enemy movements and for clearing objectives for the tactical squads to hold.

It is worth considering some rhino and razorback additions to the army to help with redeployment during the game?Basically start with a reserve group in razorbacks and rhinos and build from there if it works on the table.