Just some ideas that have been bandied around the table at my house. A couple more serious and one that's mostly a joke.
Underbrush
Underbrush hexes are filled with dense vegetation rising no more than a couple of meters off the ground. Too low to affect BattleMechs, this brush can still impact infantry and vehicles. Underbrush is subject to fires and clearing as though it were a woods hex of the same type (which stacks with any woods that also occupies the hex). Underbrush is considered to be one half-level taller than its underlying terrain for line-of-sight purposes. 'Mechs cannot attempt to uproot underbrush for use as a club.
Light underbrush - 'Mechs treat light underbrush as though it were open terrain. Infantry and vehicles (except VTOLs and WiGEs) however are affected by light underbrush as though it were light woods. WiGEs can move through underbrush hexes as though they were open terrain; VTOLs ignore underbrush unless grounded. Line of sight to a prone 'Mech is drawn through underbrush as though it were a vehicle.
Heavy underbrush - 'Mechs treat heavy underbrush as though it were rough terrain. Infantry and vehicles treat heavy underbrush like heavy woods.
Special case - Combining woods and underbrush: a hex which contains both light woods and underbrush of either type is treated as light jungle; a hex containing heavy woods and underbrush is treated as heavy jungle. When rolling to see if a hex that contains both woods and underbrush catches fire, roll separately for both the woods and the underbrush to reflect the increased flammability of such overgrown terrain.
Parkland
Parkland consists of loosely-space trees devoid of any growth below the canopy. The open spaces enable vehicles to pass through but still restrict the movement of 'Mechs. 'Mechs treat parkland as though it were normal light woods. Vehicles and infantry move and draw line-of-sight through parkland as though it were open terrain, including double damage to unarmored infantry caught in the open. When using the rules for clearing woods, parkland has half the terrain factor of light woods.
Cemeteries
This one was a bit of a joke but I still thought it interesting enough to post.
Cemeteries are treated as open terrain by non-infantry units; infantry treat cemeteries as fortified hexes. When a 'Mech enters a cemetery hex, roll 2d6. On a result of 10+ the hex contains an open grave, and the MechWarrior must make a piloting skill roll at a -1 modifier to avoid tripping upon leaving the hex.
Sewers
Any road (but not bridges) can be designated as having an underlying sewer which runs parallel to the road. This sewer is considered to be one level below the base terrain. Unarmored foot or jump infantry, PA(L)s and Light battle armor (except quads) occupying the road hex can spend 1 MP to enter or leave the sewers. While in the sewer system, these units cannot be attacked by units on the surface, only other infantry that have descended into the sewers. Infantry fighting in the sewers are considered to be inside a medium building, and only have line of sight to units in their own hex.
Sewers do not have a CF and can only be destroyed if the base terrain is displaced through damage. Any units trapped in a sewer which is destroyed in such a fashion are lost.
