Well the tidewall fortifications really fit that role quite a bit. I'd say a heavy tank that can carry the heavy rail cannon would be an obvious choice.
Finally I want to see a dirt cheap transport that is open topped and can carry up to 6 soldiers.
I believe I saw some converted piranha chassis' for this purpose.
From GW I would love to see a hovering flyer stealth attack spaceship. It would function like a stealth Apache Gunship. Kit would make two variants one with more firepower and one that has reduced firepower for 5-10 person troop capacity. I think the Commanche Gunship in real life is the perfect analog to Tauify.
I mean the Manta can hover....
Also in a weird twist of fate I want to see the Tau get an actual dedicated artillery platform. It would basically be larger more powerful smart missile system. 4d6 shots. Str 6 ap 4 with the usual sms rules. Can be upgraded to carry a pair of destroyer missiles for 15 points each. This would be a Tau MRLS system. Can be upgraded with HEAT rockets that replaces its weapon with 1d6 shots Str 7 ap 3. Both have a range of 72 inches. The vehicle need to be deployed and can be rammed and destroyed like the Stormsurge. I'm toying with giving the base anti-infantry weapon an upgrade to upgrade its rockets to plasma fire so basically super napalm rockets but can't think of rules.
I don't really see what differentiates it in role from the Storm Surge.