New Campaing
Aight, going to address these point by point:
1) Why not end the Spinward campaign in a success? The Severan Dominate can't hold out against the Imperium forever and similar rebellions have been crushed in time. Add one or two sessions for the grand finale, maybe, and give them a chance to contribute to winning. The best reason for reassignment, after all, is when the problem has been dealt with.
2) They're a recon regiment, so really unsuited to trench it out vs orks in the first place. On the offensive, give them recon jobs, determining ork positions etc. Their primary opponents are probably going to be Kommandoz and gretchin scouts/snipers and feral squigs or hunting squigs accompanying said gretchin. On the defensive, Orkz are fairly apt at warfare themselves, and with an enemy food supply in reach, their first action is likely to send in napalm-grotz to burn it to ash, as well as sabotage other key facilities. Hunting down tiny green suicide bombers in a civilian area could be an appropriate task for a recon regiment.
3)
>Should the players met said tech priest before that incident?
Yes, definitely, especially if he's the only proper cogboy on the planet. His primary task in an invasion, like any citizen of an imperium, is to aid in the war effort. He should be coordinating maintenence and lending personal attention to anything of vital strategic importance.
>know that such facility exist (without knowing it's purpose)?
I see no harm in them knowing the mechanicus facility exists, especially if it is assumed to be benign. It's not like anyone actually checks what the mechanicus are up to, and most people probably think of it as the tractor repair stop.
>Should heretek (let's call him that) experiments were succes and he created human-tyranid which is loyal to imperium?
If he's using genestealers? Hell no. Genestealer DNA is highly infective and takes over the host like a virus to create hybrids. The idiot is practically making a proper genestealer cult and arming it, without even being aware.
>Should some time pass between ork invasion and nids invasion or tyranids should attack while orks still wreck things on planet?
I would set a fixed time for the nid invasion and leave it up to the players' ideas and the varying success of their recon missions to determine if the orkz are properly dealt with in time.
>Should they have any chance to win, wait for reinforcements or i should make this agonizing evecuation when thousand will die?
All of these options, depending on how smart, stupid and lucky they are.
Just another quick question since i have no time to reply atm. What about numbers? I was thinking that major city population (which players will defend) is about 15-20k since most harvesting is automated and planet 2,5 billion population are spread along vast distance. For orks i thought that 50k orks should be a reasonable number while only around 10k are assaulting the city. Or maybe these numbers are too low for 40k scale?
Any suggestions?
It sounds a bit high, actually, depending on how many Orks you want planetside. 50k Orkz means several ships, which brings in orbital bombardment and such shehanigans. On the other hand, if it's a regular ork outbreak (from an infestation you can't quite burn out; which makes sense, given it's an agriworld) on the planet, the initial number is also going to be low, but steadily increasing once they get a decent waaghfield going. Local orkz would be what I'd personally favour, since it lets you take farming machines, loot them and turn them into ridiculous murderbots for the greenskins
With heavy automation on the planet, I would lean towards evolving mekkboyz.
Yeah, Genestealer-human hybrids are an actual thing, so i think it would be better than the technpriest knows this, intentionally creates a genestealer cult but keeps them in stasis in an effort to control them, but obviously gets in over his head and fails to keep it in check
I think unless the players are on top form with exterminating the Orks, the 'nids get there during the Ork invasion, would make for a very chaotic campaign, with the guard trying to evacuate the planet's population as the whole planet goes to absolute ****. Generally most tyranid invasions end in complete destruction of the planet. If you have a group who are cocky and think they can't lose this might be a great time to put the fear in them.
As the above poster mentioned, local Orks stealing and repurposing agriworld tech sounds ****** awesome.
but yeah have the players meet the (soon to be) heretek, you want the players to become slightly attached to him rather than you dropping the reveal "techpriest steelus was making a genestealer cult all along! and they brought the tyranids right to you!" and then have you players be like "who??? wait who cares TYRANIDS"
sorry for the disordered reply i was just writing whatever came into my head