This might not get liked by some people here but;
The micro-transaction system you had in place for ME3 Multiplayer was perfect. The Random Number Generator [RNG] system of unlocking weapons, characters, gear etc. works extremely well because it ensures people keep playing/paying for that ultra-rare they want so bad!
The RNG system works because it is like a form of gambling. People that spend money on packs might not get what they wanted and may feel cheated but there's always that idea that 'the next pack will have it' so I can understand why people bought them. I personally only bought 1 or 2 packs with leftover MS points but some people out there would have essentially bought their manifests.
The main bonus to this was that ALL MULTIPLAYER DLC WAS FREE! This is why this system worked! It ensured that you had the money to develop new content for the game at no cost to the players who put in the time and effort to max their manifests without purchasing packs. Thus not segregating the multiplayer community between those that had the DLC and those that didn't want to spend money for a couple of maps/characters/weapons etc.
So, for the next Mass Effect's inevitable multiplayer (which was surprisingly great by the way!) keep the RNG unlock system if it ensures that multiplayer DLC remains free. Some people here won't agree with me but I understand why it's there and accept it. Good job Bioware on fixing balancing issues continuously too, keep it up for the next Mass Effect. Maybe include a patch-notes section that you can access in-game to see the new changes.
Also, give us a 'shooting gallery' that we can test MP builds on that includes;
-A 'damage scoreboard' that tells us EXACTLY how much damage we are doing with our guns/powers.
-A timer system to measure DPS (useful for machine guns)
-Lets us choose an enemy to face (could be a hologram or something, enemy doesn't attack)
-Gives us options over whether the enemy is moving or the difficulty level the enemy is on.
-Scoreboard should show single-shot damage, DPS, Damage to health/armour/shield/barriers (individually), power damage, damage based on a number of factors (e.g. Geth Infiltrator cloaked, weilding Javelin and spec'd for sniper damage should all be calculated), location damage (e.g. headshots/bodyshots).
This would help in testing MP builds and might also assist in balancing too!
Sorry, this ended up going on longer that I expected but that's just a couple of my thoughts surrounding how Multiplayer should be tackled in the next Mass Effect.