1) Имхо, в сюжете место действия должно быть размером с систему(как в Белеро, например).

2) 3 Легиона: Гвардия Смерти(или Железные Воины, Имперские Кулаки), Дети Императора, Гончие Войны(Предлогаю взять Пожирателей без Примарха, бэк вроде как позволяет), можно еще взять Лунных Волков или Железных Рук.

3) Противник: Человеческая раса, использует ксено-технологии, кибер-модифицированных рабов, наемников ксеносов. Общество разделено на касты.
Преимущества: Большой флот(много маленьких кораблей(вооружение слабое), большие корабли-ковчеги для перевозки войск(пустотные войны ведут преимущественно с помощью абордажа), несколько линейных кораблей. Большая армия(очень много рабов, небольшое количество специализированных воинов, наемники, боевая техника(можно добавить шагателей или тяжелую артилерию). Присутсвует орбитальный форт(пункт 5).
Слабости: Кастовая система, рабовладение, (можно добавить орков).

4) Примархов не надо. Если уже Примархов, то Мортариона. Но тогда эту маленькую империю разнесут в миг. Можно взять одного командира(Например Лорд Коммандера Детей Императора, он планирует и составляет операции.)

5) Планеты(имхо): Нужно одну с экстремальным климатом(кислотная среда, радиация и т.д.), предлагаю добавить орбитальную станцию времен Темной Эры технологий(смесь орбитального форта, тюрьмы и улья), один мир-улей(местную столицу, тут все понятно: замкнутые помещения, помойки, шпили) . Астероидный пояс(дополнительная полоса обороны).   

6) Численность Легиос Астартес: Гончие Войны: 15 тысяч, неплохой флот(несколько боевых барж, много кораблей экскорта). Дети Императора: 10 тысяч, хороший флот(баланс между линейными и кораблями экскорта). Гвардия Смерти: 20 тысяч Астартес, маленький флот(мало кораблей экскорта, много линецных,можно пару линкоров)

7) Оружие(имхо): тут только Форжа,а вот про оружие мятежников стоит подумать.

Ultramarines (XIII Legion) - 250,000 Astartes; the Ultramarines were the largest Legion at the start of the Horus Heresy
◾Sons of Horus (XVI Legion) - 130,000-170,000 Astartes during the latter days of the Great Crusade; approximately 70,000-110,000 Astartes following the losses sustained during the Istvaan III Atrocity. Heavily engaged on the surface of Istvaan III, it is estimated that some 30,000 of the Sons of Horus were dead or unaccounted for in the aftermath of that battle.
◾World Eaters (XII Legion) - 150,000 Astartes estimated at the XII Legion's peak. It is commonly estimated that of all the Legions that had fought at Istvaan III in the purge of the Loyalist faction within their ranks, it had been the World Eaters who had suffered the greatest casualties, with well over 35,000 World Eaters Legionaries believed to have met their deaths on both sides. Aside from the many wounded, it is recorded that a number had succumbed entirely during the protracted fighting to an insane bloodlust and had to be forcibly restrained and removed back to the World Eaters fleet for containment.
◾Word Bearers (XVII Legion) - Exact numbers unknown; approximately 140,000 at the time of the Drop Site Massacre, though following their rebukement at Khur, the XVII Legion had grown to far greater strength then was originally reported.
◾Blood Angels (IX Legion) - 120,000 Astartes
◾Iron Hands (X Legion) - In excess of 113,000 Astartes at the start of the Horus Heresy. Following the disaster of the Drop Site Massacre on Istvaan V, the X Legion was reduced to a mere shadow of its former strength. Exact estimates are unknown.
◾Imperial Fists (VII Legion) - 100,000 Astartes
◾Death Guard (XIV Legion) - 95,000 Astartes at its peak. With the conclusion of the bitter fighting on Istvaan III, it is estimated that the XIV Legion had been reduced by well over 25,000 casualties, counting both the stubborn Terran Loyalists, betrayed to their deaths in the ash-mantled ruins of Istvaan III's Choral City, and Mortarion's sons of Barbarus whose lives had been spent to purchase their demise.
◾Salamanders (XVIII Legion) - Total number unknown; approximately 89,000 Astartes at the end of the Great Crusade. 83,000 Astartes deployed with their Primarch Vulkan to Istvaan V. During the Drop Site Massacre, the XVIII Legion suffered nearly a total loss, rated by some sources as high as 98% of the Legion's Space Marine personnel. However, other detachments such as the garrison maintained at Geryon Deep and the Castellan of Prometheus' forces were comprised of about 2,000 to 3,000 total Astartes, alongside the full intake of Salamanders Neophytes in training on Prometheus. There also were several line companies of the Salamanders Legion on detached deployment elsewhere at the time of the Drop Site Massacre, leaving the total number of the XVIII Legion's effectives after the Drop Site Massacre unknown.
◾Raven Guard (XIX Legion) - 80,000 Astartes during the latter days of the Great Crusade; approximately 4,000 Astartes survived the disastrous Drop Site Massacre of Istvaan V.
◾Emperor's Children (III Legion) - 110,000 Astartes during the latter days of the Great Crusade; approximately 50,000 Traitor Astartes survived the Istvaan III Atrocity. No reliable account exists of the fatalities suffered by the III Legion during the excision of its Loyalist elements on Istvaan III, but it is believed to have been proportionally very high.
◾Dark Angels (I Legion) - 20,000 Astartes comprised the I Legion during the latter days of the Great Crusade.
◾Thousand Sons (XV Legion) - 10,000 Astartes during the latter days of the Great Crusade; 1,242 Traitor Astartes survived the massacre of the Thousand Sons by the Space Wolves during the Burning of Prospero to escape to the Planet of the Sorcerers in the Eye of Terror.
◾Night Lords (VIII Legion) - At the time of the Drop Site Massacre, the Night Lords were estimated to comprise slightly over 90,000 Astartes, though others put their number closer to 120,000. Following their campaign against the Dark Angels in the Thramas Sector, their number had dwindled to an estimated 10,000 Astartes which comprised the VIII Legion during the remainder of the Horus Heresy.
◾White Scars (V Legion) - The V Legion is believed to have comprised only 7,000 Astartes at the end of the Great Crusade, making it one of the smallest of the Space Marine Legions, if not the smalle