Well i'm bad with coming up with appropiate point costs. Only thought about the worser save and slower speed with lack of Indirect Fire  compared to Whirlwinds.

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Why wouldn't it have indirect fire?

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It can't in Wh40k. It's a direct fire only weapon.

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It' more like a light vehicle in 40k. And no, it doesn't have indirect fire.

I still wouldn't have them as formations! It feels very un-space marine!

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Agreed - a single one (perhaps at most 1-2) as a supporting upgrade to a Devestator formation would be a better way to go than a separate formation. That would be an interesting extra boost that might make fielding them on foot more viable, while not altering the overall list interactions too much.

As I noted before, I reckon an Imperial Fists list would be a great place to have the Thunderfire Cannon.

It could also have trenches, and a siege Dreadnought perhaps:



Maybe Tarantulas:



And Land Raider Redeemers for burning heretics out of their trenches:





But no Thunderhawk, Landing Craft, or Spacecraft!

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mmm...redemption! A siege marines list does sound a novel idea; and one that allows for a more than 'we use thunderhawks and titans' type of play for marines.

Although has anyone else noticed GW's current obsession with siege war

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Well, Krieg has them making money hand over fist so why the hell not.

But please don't make Imperial Guard marines out of Imperial Fists. They're good at sieges and urban combat but that's hardly the only thing they do.

Given that they're stricter adherents to Codex Astartes than Ultramarines (no, Cassius, you can't have Tyranid hunter units) they really don't need any other list than Corebook Space Marines. Let alone leaving them slogging around in the dirt like that.

Final idea: have drop podding thunderfires as upgrades to podding formations. That's the only way I can see their deployment making sense for Space Marines.

Maybe have them deployed by the Thunderhawk Tranporter? That seems a much more likely method to get them to the surface rather than having to invent a much bigger drop pod for them to come down in.

I do think they can use standard drop pods. But since I have neither model in my 40k (Imperial Fists ) army, I can't see if it physically fits. But it's probably not a lot bulkier than a dreadnought.

IIRC they can use Drop Pods in Warhammer 40,000.

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In the novel "Salamanders" the Salamanders used several Thunderfire Cannons to help to defend their crash landed Strike Cruiser.

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Ok Drop Pods it is then (sorry, not that familiar with them).