Consider, Straken for BN HQ, at least one platoon of infantry (others might be meltavets or such) then add in the EC, two 12-model Crusader squads (assuming they get frags and better setup) and maybe a Sword Brethren squad (again, assuming GW fixes them). The idea is that Straken will confer his Furious Charge and Counterattack to our troops, plus the EC will give us "hatred/rage" (+2A in CC, what I'm fairly sure we are getting) while the rest of the IG provides the base of fire for the assault. Even toys like a Speeder squadron and a Dreadnought could be tacked on. The key is if the EC becomes a "special character" or not for the purposes of being allies, because if not, then we can benefit from some other codex's named characters by making them the "primary" army.
EDIT: <sigh> The rumor around the Net is that you cannot ride in your "brothers in arms" rides, even though you share special rules.
IG driver: "Lord Champion of the Emperor, hop into my vehicle and I will take you and your men to the <relic/targeting array/shrine/shield generater/other objective> before all is lost!"
EC: "Nay, good Guardsman and loyal servant of the Emperor, this is forbidden to us! We shall strive our best on our own feet to cover the impossible distance!"
IG driver: "Milord, you must get in for even your mighty physique, long training and formidable powered armor will not let you cover such a distance in the short time left to us!"
Chaiplain: "Allow me Lord Champion!" EC nods.
Chaplain: " Brave Guardsman, I know your heart is in the correct place, but you veer dangerously in your beliefs! It is written in the book of the first High Marshall as handed down by Dorn from the Emperor thusly: 'Thou shalt only ride in the Rhino and Land Raider. Thou shalt not ride in the Chimera or Repressor. The Chimera is not for thee, but only the Rhino. You will be let slide on the Stormraven, though the Thunderhawk is the only proper flying ride. The Drop Pod is also allowed thee, as well as to thy Dreadnought Brethren. The Valkyrie and other transports are not for thee, thou shalt not ride in them, on them or even walk too closely to them, they are denied thee.' Thus the words from our Emperor, Primarch and First High Marshall."
IG driver: "But my Lord Chaplain, this is to save the Imperium and for the Emperor!"
Chaplain: <pulling his crozius from the Guardsman's skull> "I am sorry for that, but I have saved this brave warrior from the taint of heresy. Shall we run brothers?"