How about either of the following?
Not to spam suggestions, but I've always thought the guns on the APCs from 40k (the actual XCOM crafts) would be a PERFECT fit for either the mudranger or the humvee. Thoughts on implementing such a functionality? Having a heavy gun like that would go a long way towards making some landing sites more manageable, and perhaps even sensible considering every xcom craft mission landing is borderline suicidal 
The thing is, how you deal with the ambush is part of the game. Smokes works wonders, smoke the ramp before leaving, also just letting the first turn just pass so enemies can scatter and lose TU to prevent reaction fire before you leaving your vehicle.
With tanks it will be easier since you can roll them out first and let them tank the reaction fire. But the most help will be from certain landing crafts. Not all have one ramp. Some have side doors you can pop up from (and these doors always close at the end of the turn so you can spray and pray and end turn with 0 consequences from the enemy since your troops will break LoS). Some have staircases...
Some even have a benefit of being above ground so you can hide your men beneath it to avoid long range grenade snipes across the map.
For example, Skyrmarshall has 2 side doors alongside the main one.
Ironfist has a staircase to the roof and you can use the staircase to throw grenades out of it. And you can also stand beneath it.