The warhound would be a multi-purpose weaponry unit with a low amount of hit points, very akin to the tank, but with anti-air capabilities and a twist to it's weaponry. There would be 3 weapons total.
Gatling-kind of weapon for range 0-5 to give at least a small help against things getting close, not ideal range. I see it stealing the viking Gatling (not in term of damage and attack speed, just the type of weapon). I think the landed viking could use a buff to differentiate his weapon from the "new and improved" warhound. The change could be piercing bullets, all units in a line of 3 behind the unit hit by the viking would also take full damage (this really isn't much at all... they would be better support against small massed units, at least that's the point, maybe not what it needs, but still of some use). Maybe also decrease time for it to get back to safety (flight mode), this could be an upgrade. I would definitely like to make armies composed of more vikings. I can also see banshee getting a buff because besides for having a small window of time where they are effective at killing scvs or surprising an opponent with low anti air firepower, they aren't that good in actual battle, which ISN'T fun. Units should be fun to use and not too focused on one thing... I've always wished for HighTemplar/Infestor/Raven to have their own attacks and decrease impact of landing spells. Back to point now.
Mortar-kind of weapon for range ~7-11 (pulling those numbers out of my !@# here) that would target the FARTHEST units. It would deal a small AoE attack with medium attack power and be the ideal range for this unit. The point to making it attack farthest units is to : attack units fleeing from your army while being able to move your own toward his, by reaching the farthest units damage is spread from the rest of the army, weakening but not killing the opponent's army (in theory, if you mass too much your army gets weak in close range firepower and the opponent will just charge you); reach for swarm hosts instead of their locusts, implying they burrowed close enough, just get vision of them with a raven or something.
Missile kind of weapon with a minimum range, leaving it vulnerable to any close air thing but capable of being fired simultaneously with a ground weapon. Thor is a good anti air unit, but now it needs 4 seconds to switch modes. The missile would hit a single target, contrary to the attack of the thor. TANGENT: I think limiting amount of target hit by a single anti-air thor shot to 5-6 would be balanced... the poor clumped mutas suffered enough and speed doesn't fix the fact that you got in the range of many thors by mistake, although it's welcomed for the increased harass power (there again making the unit more engaged into a single role, which I'm not sure I like).
I think you can see this unit has potential, hopefully without being OP. And would be fun to use. Effectiveness in bio would be hopefully be decreased by the combination of cost and fragility factors (I consider tanks and hellions (not hellbats) quite fragile). Their most effective place would be near tanks, where units do not want to go, while being able to force confrontation from a distance, giving Terran it's own way to force engages, akin to the zerg's swarm hosts and protoss's tempests. Giving them this possibility is only fair. And the composition would only force you to use units that can deal with tanks head on. For example ultralisks, once getting past the mortars, which should MAYBE be a projectile, would suffer nearly no damage from the Gatlings, and we all know they rock tanks once they can get close enough. Warhounds would then either slowly back away, having legs instead of wheels (stupid engineers!), leaving you to kill tanks and getting hit by a few salvos of mortar again, or take hits and let the tanks either shoot their destructive siege-mode attack or unsiege and flee to the base.
They could also be used in conjunction with thors, thors giving the warhounds vision and protection. Giving the Terran player a powerful army, but slow/ly moving/built and costly in both money and supplies. You would also need to fear surprise bu.. s.. I mean being attacked by behind....
Their weakness would be being mixed in bio because they'd be easily taken out by a ling/baneling, chargelot heavy protoss army or enemy tanks or even stimmed bio out of mortar range (inside the "blind spot").
INTERESTING RIGHT? Well I know some answers will disappoint me, this can't be helped, I already can see the QQ of having a unit with 3 weapons. I will further explain my point of view if clarification is needed/made no sense in what I wrote.