For SOASE 2 Combined Spacebattles with DETAILED Ground combat would be the win. It's completely doable I think.
Make space bombardment have lasting consequences on a planets value so invasion (especially of terran type) planets is useful, make structures capturable. Give planets landbased structures (eg space elevators would increase the economy but cost a lot to build). Universities would increse the speed of overall research (by a small ammount per University, multiple universities on a planet would have a synergistic effect however). Space Academies would result in more skilled ship crews coming from ships produced in that gravity well, strike craft produce in hangars in system would also be better. Troop Training Centers could allow troops to be produced there, Army Academies would result in more skilled ground forces produced on planet. These would be expensive and take time to build (ie enough to make a planet more desirable to capture/bombard) Underground Fortresses make a certain ammount of your civilian population invulnerable to bombardment until the entire planet is killed. Farms would increase economy a bit, and allow larger populations, as well as produce excess food that can be transported to other worlds (or traded to aliens). Mining Centers would produce resources on a planet (planet based resources would be more variable than asteroid, and they could change in value based on planetary morale, loyalty, goverment type, and good or bad luck. They would have a negative effect on food production and morale however. Factories would increase the value of trade produced on planet but also affect morale. They could also increase the production speed of units and structure produced there (synergistically, ie one factory 5%, two factories 15%, three factories 30%, etc)
Each of these (and other) structures would cost a certain amount of planetary points, some could only be built on certain types of planets (eg Terran would be best for farms, though they might operate at 25% on Ice, mining centers best on Volcanic etc)
Your ground forces would fight fairly intelligently on planet, and you could drop in to give them more specific orders if needed. They'd be landed by dropships and consist of three basic classes (Infantry, Tanks/Mechs, Capital Ground Units (ie Superunits) These different classes would be researchable to upgrade or get new units available. Each unit would cost an overall army point value (limiting total army size in a manner similiar to fleet sizes in SOASE 1), in addition it would have a transport value (limiting the number that could fit on a transport vessel (or the types of transport vessels that could carry it). ALL units would gain experience and go from green to elite. Capital units would be capable of firing at ships in near orbit (as would some specialized smaller units). Strike craft could be used to support/defend ground forces.
You couldn't build your own forces on the planet until you've captured at least a continent (planets could have multiple random continents with varying terrain in each region, troop construction would be slow unless the entire planet is controlled by a player (based on percentage of planet owned) , in most cases, ground troops have to be transported in.
Get new classes of ships.
Troop Transports (frigate, cruiser, and capital sizes), Teraforming ships (capable of transforming planet types), large Cargo, Mining, and Construction ships (their presence would increase construction speed or improve the economy/resource gathering, but cut down on your military force), Food transports allow larger populations to exist on less habitable worlds (but if those transports don't arrive in time...)
These would affect your economy, military capability, research, and more, they would also affect aliens relations with you. Democracy, Monarchy, Dictatorship, Fascism, etc.
Allow more options, eg demand tribute, Join the Empire (pay your goverment taxes), form trading blocs (you get a separate "black market where prices might be cheaper for some goods), form defensive pacts (you contribute ships to neutral fleets that act as quickly as possible to defend pact members, if a defensive pact breaks down then those ships may turn pirate, may join various ex pact members fleets, etc. You might be able to bribe their fleet to join yours, Assasinate their leader, support them economically in order to defend yourself against whoevers on the other side of their gravity well.
These would be similiar to the neutral forces on planets before they're captured, but they could act and react diplomatically, and they would be researching Space Lane travel (if they discover that technology then they become a major faction and can expand, research, etc as starting factions. They can also be asked for tribute, to join the empire or else, react to your culture, suffer ground invasions and be forcibly integrated, etc.
Sneak a nuke into a Novalith Cannon? Capture the plans for an enemy ship? Steal technology? Assasinate a planetary Governor or Assasinate the Emperor (resulting in enormous economic and cultural damage for a period of time either for a planet or the entire empire) etc. You'd need a specialized Academy to make spies, and they would gain experience for successful missions much like capital ships, getting skills related to their intended role (Sabotage, Assasination, Subterfuge, Technology theft, inciting rebellions, etc)
More Variability in planets.
Planets should be unique, and have multiple features that set them apart and increase or decrease their overall strategic value or interest. Exploring the planets should have more wonder and surprise.