My dream game? An RTS/4X TBS strategy hybrid where you have command of both ground and space forces. To have an idea of scale, think of a cross between Sins of a Solar Empire for the space combat, World in Conflict Wargame: European Escalation for the ground combat, and Earth 2150 in terms of research and unit design.
The space combat would more or less play out exactly like it does in Sins: you shipyards, fleets, perform research for upgrades and newer weapons, etc. There would be two key differences though. First, in order to take over a planet you'd have to wipe out any forces within a certain range of it in order for your invasion forces to land safely. Attempting to "Force" your guys through would result in a percentage of the invasion forces getting killed before landing. Also the enemy might still have ground-based orbital guns to shoot down your ships, which means you'd have to decide between tolerating the planetary fire until you're ready to invade, or bomb the guns while risking damage to the planet's infrastructure.
Ground combat would be a mix between World in Conflict and Wargame: European Escalation. You'll start out with one "Safe" landing zone and call in your forces there, with the goal of destroying the enemy's command unit. You can also land your forces anywhere on the rest of the map, but you can only call in so many forces at a time: therefore, you can't call in all of your guys at once. Also, the enemy can build some AA missile units capable of intercepting and instantly shooting down your transports, as well as build radar sites to detect where exactly your forces are landing. That would prevent you from surrounding and rushing the enemy's most likely command positions.
You'd have about 4-5 unique races to choose from, each with a tech tree of various uprgrades and weapons to research. The units aren't restricted to using one weapon type either: for example you could change out the ammo types a unit uses, give it a better engine to improve it's speed, or even change it's main weapon out. The same goes for the starships as well: you wanna put a stronger, more power-consuming engine on your battleship to improve it's agility at the cost of having less punch with the guns? Or design your fighters to carry a variety of weapons while at the same times staying just as manuverable, but at a much greater expense? Go for it!
You could also mix-and match some of the unique techs from the different races. For instance, if you were to capture the tech of a race that relies entirely on organic units (read: Zerg), you could then apply some of their regenerative abilities to your infantry, allowing them to heal on the field. Capture the tech of a race that has point-defense lasers, and you could start building them for yourself. Find the tech for gauss cannons while playing as a race that's proficient with zero-g tech? Then you can start building your own mass-effect style guns!
Multiplayer would be the tricky part, though. Can't really think of a way this system would work well online without becomnig a drag. A lot of players wouldn't be interested in how long the matches in this sort of game could get, unless they don't mind continuing from save-states.
I probably could type a whole essay on how I would design my dream game, cause I've got tons of more ideas for it... but I think I'll stop here.
Would you believe I never fully watched the original Indiana Jones trilogy? I gotta correct that someday.