Proximity Mine is very powerful.
It's been so long, but something like
this. 6/5/6/6/3
Your health is squishy, but your shields are very strong so an emphasis on them is worthwhile. This isn't the outright highest Javelin setup, but it lets you shoot through the map which is very fun. I don't see an option for it, but Warp Ammo IV packs a punch.
This is all assuming you go sniper of course. Different build entirely for shotgun and scorpion melee.
ME3 MP nailed 'less is more' so hard when creating multiple viable builds.
Thinking on this point... Favorite loadouts are going to be weird if we can or can't (both) have different power evolutions for different situations. In the example above you would spec the same powers differently for sniper, shotgun and melee builds. I wonder how that is going to play out.