I'm just going to cover what TANKS I'd like to see in the Tanks DLC. Following the pattern of the Jets DLC, we have one for each faction, then a bonus vehicle.
First, CSAT. This has been hinted to be the T-14 Armata, and what a great choice it is. But give it the proposed 152mm gun. Why? Because it would be boring if all the new tanks in the game had the same gun as the present tanks. Also, having a platoon of these guys roll up on you would be the indicator that everything is being turned up to 11. The T-14 Armata has up to date electronic warfare and ACTIVE armour protection fitted, as well as ERA, Composite and RHA.
NATO. You could put in the Abhrams. But, as with the choice of guns, this would be boring. My choice would be the AMX Leclerc. It's a bit left field, but one of the best tanks in NATO atm (I know there's a lot of 'best tanks' to choose from in NATO), but it is the one of the first tanks to actually have a 140mm gun fitted to it (to counter the T-14s 152mm). It already has a three man crew, and has armour designed to be effective against both HEAT and sabot (composite, but not chobham), and has the ability for electronic counter measures. Also, just give the Slammer UP an 120mil already.
AAF. With them already having the Kuma, I feel more inclined not to give the AAF another MBT. After all, their equipment is NATO hand-me-downs. Just give it a few upgrades in terms of active armour and electronic warfare. Possibly the new Rhemintal 130mm gun. What I'm more interested in is a possible light tank or SPG. The former would be the M8 AGS, the latter the M1228 MGS. Both have 105mm guns and excellent mobility. The idea of introducing these AFVs is to have something to support mechanized infantry and IFVs that isn't a tank, or be part of a QRF (both M8 and the M1228 are designed to be air dropped), or direct fire support for a small infantry force. It would be nice to have something that fits the gap between IFV and MBT. Out of the two, I'd favour the M1228, as it is actually in service atm, and it shares the same chassis as the Gorgon.
The bonus vehicle. Something for the FIA and Syndicate to use. Something that would make you go "Errrr, they've got a tank" when your on a spec ops COIN mission. Nothing fancy, but still a threat. The T-34 and M4 Medium tank are touted out, but by 2035, they would be REALLY long in the tooth, (and require a four man crew, and would want to be the loader?). My choice would be the AMX 13, either the 75 or 90 autoloading gun (thus a three man crew). It's a tiny tank, barely taller than a man, so easy to hide in bushes or in defilades. A great ambush tank, and highly mobile. Its also thinly armoured, so the cannon mounted on IFV should make quick work of it if spotted. But equally so, the 75/90 guns should be able to deal with IFV, and could worry an MBT if it got around the back (think the Tiger battle from Fury here). And it would also fit on Altis or Tanoa, since it's feasible that a newly independent Altis/Tanoa brought a bunch from the French in the 70's, and somehow they got into FIA's/Syndicate's hands.
Some of the features I'd like would be, as hinted at above, an actual armour model, that takes into account different projectiles and armour compositions. Electronic warfare capabilities for the MBTs, so you know when you are being locked on by an ATGM, and use electronic counter measures. If that doesn't work, active armour protection, so the incoming ATGM is shot down before it hits the tank, automatically (but you should be able to turn the system off, so that if friendly infantry are close, the system doesn't kill them). Also, more than one periscope for the driver when he has turned in. And a reverse camera. Better physics and tank AI would be nice (people have already covered that in more detail that I can do).