Sorry if this has already been suggested, because if so I couldn't find it. Took me ages just to find the suggestions board anyway.
So I've been thinking, if the latest versions of Minecraft have dual wielding, why can't we? Here are my thoughts on the subject.
There would be an offhand slot like in vanilla Minecraft, but you can only put smaller weapons in it (e.g. knives or wood elven swords). Your main hand in the hotbar can hold whatever you scroll onto. If you scroll onto a larger weapon (e.g. a battleaxe or warhammer) your offhand weapon retracts and is un-useable untill you switch off of it.
Holding two weapons in your mainhand and offhand you'll be able to left-click to hit with your mainhand and right-click to hit with your offhand. This way you cannot block which can be a small nerf. Each weapon retains its own attack speed.
Shields will be visible only one you have one weapon in your mainhand. Having two weapons or a bow will retract the shield to your back.
Knives are special, holding a knife in your offhand while your mainhand is empty will make the knife invisible to other players, giving the notion that you are unarmed.
Bows, Crossbows, Battleaxes, Warhammers, Spears, Pikes, Halberds, Maces, Battlestaffs, Lances, Poleaxes, Staffs, Large Swords and other large weapons should all be single handed. While Knives, Scimitars, Small Swords, Normal Axes, Throwing Axes and other small weapons can be dual wielded.
Blocks in the offhand will work just like vanilla Minecraft. Left-clicking will break blocks while right-clicking will place them.