Signed - with caveats.
The "unreleased" maps should be easy to release. New kits/weapons, not so much. All the work necessary to create (for example) a new Krogan engineer could be better spent on developing ME4. Say you have to retain a small team of developers to code the new DLC - say 6 people for 4 months. I don't know what contract developers make, but say $10k/month/person including overhead costs (office space, computer, support personnel, etc.) - that's $240k. At $20 a pop for DLC, Bioware would have sell 12,000 of these just to break even. I don't know if there are 3000 people still playing this game, much less 12,000 who would cough up $20 each.
However, I suggest another DLC would be a good opportunity for Bioware to test drive some ideas for ME4MP. I thought I read somewhere that ME4 will be built on a different engine than ME3, so I expect the programming for the two engines will not be the same (but I leave that to folks more knowledgeable than me to confirm or clarify.) We know that ME4 will still be set in the Mass Effect universe - i.e., mass effect relays, Krogans, Salarians, Turians, flying cars, spaceships, energy weapons, etc.
Some ideas can be translated across engines for some alpha or beta testing without calling it that. For example, the Scorpion pistol was a concept introduced in the cutscene on Sur'Kesh when Captain Kirrahe pulls out the new toy and blasts a few Cerberus troops with it. The concept of a pistol sized "grenade launcher" that fires sticky grenades was new to the ME universe and had to be tested before being released. Should these rounds penetrate shields/barriers, or armor or both? Should these rounds penetrate a biotic sphere (i.e., phantom, Praetorian)? How much force should each charge impart? What happens if you equip the pistol with incendiary ammo, explosive ammo, or phasic ammo? What constitutes a "head shot" for this weapon? Suppose they want to introduce a taser pistol with effects similar to the Juggernaut's heavy melee. It could be a great alternative to the Acolyte for stripping shields/barriers, but for balance, it could not last as long as the Juggy's heavy melee. Graphically, it could be a juxtaposition of the Rheegar carbine and the Juggy's heavy melee, range of 1-2 metres, max. Initial clip may be limited to 3 second burst - enough to stun a Phantom, but not enough to kill her outright. So you play around with the existing pistol graphics, come up with a new name, and imbue it with some existing powers from other elements of the game. Voila - new beta taser pistol.
If Bioware is considering new weapons or new powers for ME4, ME3 is an established game with a lot of players with excellent game playing skills and, more importantly, a lot of players who understand gaming concept, tactics, balance issues and can provide the developers with both good statistical data and quality feedback about how something does or doesn't work, and most importantly, why it doesn't work.