Implementing ability select for DW would also propably allow in future to enable combiweapons for 2H as they would most likely be able to reuse parts of already existing code. 

Melee + Pistol and Pistol + Shield are both in the game again. They're decent, depending on the combo.

Mixing and matching one handed weapons was possible in an earlier version, I remember having a Power Sword and I think it was a Laspistol. It was fun but there was an animation issue when using the Laspistol. Now, as I recall they changed this so it was paired (chainsword + shield for example) and they mentioned they were going to work on this issue and bring it back so you could freely mix and match.

If anything it's just a waiting game on getting this back, most likely at full release or perhaps just after. They do have a lot of other stuff to work on first :P

i will say the ranged/melee combo presents unique difficulties, because many of the dual pistol setups actually have animations that create two beams/shots/volleys/what have you. they would essentially have to create a third set of skill for melee/pistol combos to avoid broken animations and other such hilarity. if they intend to keep pistol/melee in game,  i think it would be good to actually set up a dedicated playstyle for it with unique abilities, otherwise scrap it. something interesting as a possibility would be something similar to FFXIV's system, where you can have actual dedicated dual wield setups...paired laspistols/blades/ whatever that have an actual 4 ability dedicated set of moves, and the regular one handers that only have 2. that way, you're not trying to balance pistol damage around 2 separate weapons, but you can maintain that feel of it being different from using a rifle, and they'd still benefit from onehanded weapon perks if such exist. then, you can give the onehand only weapons slightly stronger or more useful skills, and not worry about it breaking the dual wield setups. so a one handed pistol can have an advancing shot, and the melee weapon can have some aoe clear or single target burst like they currently do, without either hurting a dedicated melee or ranged setup. you can even pair weapon/shield combos and have them function uniquely, as a powersword/stormshield combo would be focused around assault and nimble combat, while a power hammer and suppresion shield would be focused on getting in and tanking hits while slapping elites around. the idea is you can focus on a playstyle for a certain setup, rather than trying to balance the weapons individually.