The pelican drop is the weakest of the UNSC leader powers even for its
meagre cost. When used it must be done as discreetly as possible,
especially if your enemy has a vast arsonal of quick moving attack
units (e.g. anything that isn't infantry) and even then you had better
hope that they are paying any attention to odd shadows that float near
their base. Any improvement would be greatly appreciated, even if it
meant balancing by giving the Covenant back their Spirit dropships. Possible upgrades from the base facilities could make pelicans a bit more useful then they are currently. Imagine buying upgrades that grant your pelicans increased armor, travel speed, chaft missiles, or even a gunner turret for cover fire with a snarky marine shouting a slew of Sgt. Johnson quotes.
Pelicans are best used to group your troops near an enemies base, not on top of it. A skilled player could easily stagger a flock of pelicans drop pattern so that they could land all their units at a location and have them attack at the same time.
As for missile squads, they are not a neccesary addition in the Halo Wars world. Cheaper/quicker marine upgrades would be nice addition, even if just for Cutter giving him a better position among the UNSC leaders, and a better chance against the Covenant leader rushes.
I've played hundreds of games as Cutter and won greatly to the fact that people thought that I would build infantry. Many games were just handed to me only because people would think that because I played cutter I'd build infantry. Instead I'd commonly bring warthogs and other vehicles to the field, seldomly building a barracks unless to counter a Cutter who was devout ODST with flamethrowers or for the fun of having a covenant leader meat his demise at the hands of twenty flamethrowers standing at the doorstep of his base. The most useful uses of ODST are as only support troops to hold units down, while teammates can carpet, cryo, and leader power. That support position is sadly only useful with Cutter's ODST, and only by sheer luck or an extreme amount of battlefield manuevering can any player wield Medic marines in the same manner.