I would make a Mech Warrior-esque Titan game

Well, here some of my simple ideas that i invented in several minutes:
Genre: third person action-survival-horror, something like dead space with small amount of ammunition plus some steals situations like in manhunt.
Plot: Main hero is a member (magistratum officer or arbitrator) of raid group to the low levels of hive city. The one who avoided captivity. His mates captured by chaos\xeno cultists for sacrifice. Situation exacerbated by fact that he has mistress among the colleagues. So hero wants to free his mates and bring the light of God Emperor to that pity cultists (by slaughtering their bastard faces with rusty pipe or other things that can be founded in the tunnels). There can be a lot of enemies: cultists\tribe of cannibals\humanoid mutants\some HOLYFUKKENSHITy creatures like teenage mutant ninja turtles animals\just some xeno fauna that adapted to the under-hive life conditions.
P.S. Sorry for my bad Low Gothic.

I'd make 3 games, completely different:

1. Classic remake of Chaos Gate.

2. An rpg with a huge sand box world a la Fallout 3 featuring an Inquisitor.

3. Tactical shooter\rpg featuring a war band of CSM with the option to choose the subfaction ofc (with all the consequences tied to this choice).

I'll be the boring one lol,

I'd make a computer game version of the table top game thus enabling TT players to face both a range of AI opponents (like kasperov's gambit for chess that had 100 different AI levels to defeat) as well as other online players, that comprised of all the possible TT armies/units from both GW and FW approved lists,

this would allow players from isolated areas access to TT style matches even where RL opponents are extremely scarce, where lifestyle restrictions to availible play time would be negated and where players budgets would not significantly handicap their ability to field an army for a match

I would make a game where you play as Chaos, and you click a skull, and each time you click it, someone gets corrupted. Cultist Clicker.

A game based purely on the table-top game. With all units and factions and maps that can be user made with in game tools featuring all the buildings and customizable terrain. Gameplay would be similar to Combat Mission with updated graphics and animations.

Space wolf beard simulator of course

A 40k RPG, probably having you as a role of Inquisitor.

Think mass effect meets 40k, with the possibility of the player character being put on the trial as a heretic and made into servitor if they are all buddy buddy with aliens and not grimdark enough.

Otherwise a dynasty warriors esque 40k game would be cool. I would normally also say a Total War 40k but apparently that one is actually in the works.

Istvaan V MMO. It needs to happen.

1. Make a horror/first person shooter about the "War In Heaven" or pre-40k Commorragh.
2. 3rd person shooter game with RPG elements about either the 13 Black Crusade, The Legion Wars in the Eye of Terror, or The Horus Heresy.
3. Stealth action adventure game where you play as the Deathleaper invading imperial worlds.
4. First person shooter multiplayer-only (maybe single player?) game featuring the various navigator houses of terra fighting for control over the Navis Nobilite's recourse and allies on different planets including Terra, Mars,Necromunda, and possibly even the Halo zone!