Ground Vikings are weak. REALLY weak. They weren't always so bad. IIRC at one point in WoL Beta they had 1 Armor and 14 damage and could compete toe-to-toe vs stalkers and hydralisks. Thing is, a usable Ground Viking would actually fill a lot of the holes in HotS Mech.
What do I mean by usable? I think it would be a viable unit with just two changes:
- Vikings may use either Mech or Air upgrades, whichever is highest. (this is the big one)
- Attack changed from 12 damage to 7x2 damage. This is only a 17% buff against normal units, and not a buff against high-Armor units, but is a 40% buff vs immortal shields.
Why would a usable Ground Viking fit well with Mech play?
1) You'll have a starport anyways, now that medivacs heal your firebattlehellions.
2) Vikings cover the main weakness of mech which is Air.
3) Vikings are amazingly supply-efficient. At 150/75/2 they have a 112.5:1 cost:supply ratio which is higher than anything except spellcasters and Banelings (150:1). HotS mech uses a lot of hellions (50:1) and widow mines (50:1) so you really need a supply-efficient unit to make up the difference.
4) Vikings are surprisingly good against Archons. Throw a ball of ground vikings into a ball of archons in Unit Tester. The viking's fast attack busts through shields while its bulky size and Mechanical nature makes the archon's attack highly ineffective.
With these changes, Ground Vikings would still be fairly weak and not a unit you'd really want to mass, but you could sprinkle a few into that mid-range crescent in between your hellions and tanks. They would do wonders against Archon and Immortal shields, while making you much less nervous about an air switch.
I'm not the first person to think of something similar: