Not quite that far

Something like a Tactical with a Combi-Weapon and a Power Fist?

Possibly yes

A Combi-Weapon is something I'd very much like to see, although I do see it being limited to Tacticals with a "Veteran" skill.

I would have wanted Combi Weapons in the game anyway, Bolter + Plasma Gun or Melta.

Grey hunters all have bolters and close combat weapons of some kind, whether they be chainswords, axes, swords, knives, chains or mead mugs. Are we playing Space Marines or Ultramarines only? 'Cause if I'm playing anybody besides Ultramarines, I don't care that much about what the Codex Astartes says.

Tactical marines shouldn't get chainswords standard, but they definately should be able to get them as a perk earned a little down the line. If you feel like this will ruin assault marines or whatever, give them access to power weapons and whatnot to ensure that they still remain king of close combat. Failing that, give them access to their own perks that allow them to do more damage/have a larger combo with chain weapons.

I think a tactical marine should be able to wield a bolter pistol and chainsword after achieving something, in lines of becoming a tactical sergeant.

A tactical sergeant losing some ranged power to gain some melee power while gaining passive benefits to surrounding players.

Hell, maybe the leader of any group that is full can choose to become a tactical sergeant for some resources.

I think you wouldn't see too much of an issue. MANY people don't like "leading" groups and I say "If you don't want to step up, you don't get all the awesomeness of sergeant!".