unit ideas for scifi wargame

Difficult one to answer without knowing what level of technology and general theme you are after. Far future or near future?

Also the nature of the battleground....planetary surfaces, deep space, etc?

Assuming you mean futuristic Sci-Fi, you generally have the following options (though you have/will have invented your own universe, so it will be quite different).

Space Sci-Fi: Good examples are Star-Trek, Star-Wars. These normally involve space-based combat, sometimes ending in land combat. Space ships are also used to bombard planets. A game based on these would therefore probably have many ship types, maybe even customizable, with a couple land unit types.

Near-Future Sci-Fi: This might involve many unit types still in use (some vehicles, artillery, ships, and infantry. Good mixing in all categories). If it involves space too, then some of those types would be advisable.

Dune and Ground-Control Sci-Fi: These might involve minimal to no space comabt, and some minor space support (though space control would still be important for reinforcements, if your game has any). Good unit categories would be infantry, ground vehicles, air vehicles, and artillery.

You might also want to think about your mech/normal/bio ratio. Finally, you probably want to consider super weapons. Do you have nukes, or super-units?

A good naming rule-of-thumb is the bigger and badder, the more impressive the name and interesting the background story.

I'd also add that you may want to pick an analogous conflict, and "sex it up" with SF chrome. For example, most "Classic SF" is World War II in disguise, with it's preponderance of tanks, emerging technology, terrible slaughter, and individual heroics. "Aliens", by contrast, was the American experience in Viet Nam, where superior technology was powerless when combined with strategic and tactical error and a relentless enemy. "Classic Anime", from 1981 to 1990, was basically the Pacific War of WWII: huge fleets of ships battling it out, with fighters weaving and attacking these basically stationary targets, and fleets moving between stellar "islands", or hiding among nebula clouds and so forth.

All analogies. All effective because they echo the real tactics of a given situation, and don't feel like random elements thrown together in a "salad of destruction", in which all possible units are present because they can be. Focus on one aspect of this future war, and you'll have greater success.

The air superiority model for future conflict only works if effective anti-air does not exist. If the enemy has lasers or hyper velocity rail guns, fragile aircraft are a major liability and tanks and infrantry will be center stage. Even missiles and artillery shells are not immune as point defense improves.

Assorted sci-fi man-to-man roles: infrantry, assault, recon, psi, communication (includes directing orbital weapons fire or hacking enemy systems), command, area defense (force shields, point defense, nuclear dampeners, etc.), heavy beams, rail gun crew, power armor, cyborgs, robots (vehicular or bipedal), unmanned vehicle or robot operators, grenadiers, mortar crew, bioengineered creatures and their handlers, combat engineers (providing power generation, demolitions, etc.), vehicle crew, suicide bombers, teleporting melee skirmishers, the list goes on...
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TAZ

1) Light Infantry - light armor, light weapons, very fast.
2) Heavy Infantry - heavier armor, heavier weapons, not so fast.
3) Mecha Armor - Decent armor, heavy weapons, rather slow.
4) Robot Sentries - average armor, high rate of fire, immobile.
5) Scouting drones - Lighly armored, no weapon, very fast.
6) Warmachines - sci-fi equivalent of siege engines. Heavily armored and slow, but they pack a punch.
7) Personal Infantry Carriers - carries a varies number of infantrymen. Faster than walking...
8) Satelite Guided Missiles
9) Orbiting Laser Cannons
10) B.F.G.s =D

My advice is to come up with the factions first. It's science fiction, right? Come up with the fiction! If we're going to be taking sides in this conflict, give us some sides to take!

As an example of what this does for a game, consider GW's Warhammer 40,000. The first edition was a barely-playable mishmash of ideas, and would have tanked completely (I'm almost certain), if it weren't for the backstory, and ample fictional work. This background was clearly an amalgamation of Frank Herbert Dune series and the many works of Michael Moorcock, spread thinly over a universe right out of Tolkien and made to match the tone of an Iron Maiden song. But it worked well enough to keep players struggling through games, and eventually evolved into its own fictional universe.

In the WH40k universe, Space Marines were supposedly the baddest of the bad, the ultimate human wariiors. And yet, as teh game evolved, their battlefield performance grew a bit weak, so the designers changed the rules to make the game fit the setting. This is good design, in my opinion. People understand and relate to the background story far more than to abstract game stats. I'd suggest you do the same. Once you have your history roughed in a bit, you can decide the technologies and tactical bits that will make up the army lists. Put balance and "typical SF units" by the wayside. Make up a new universe, and then give these forces science to fit their fiction.

Space Miner's Confederation: Also known as the Roughnecks, this hardened group of blue-collar asteroid miners originally started as a small, isolated worker's strike. Rebelling against their parent corporation, Galactech, the SMC strike quickly grew to a massive union revolt, bankrupting Galactech and disrupting the raw materials trade. Utilizing retrofitted mining equipment and vehciles as weaponry, this piecemeal army has become a force to be reckoned with.

The Bloodless - Bent on immortality, the Bloodless are a cryogenically frozen sect of humanity. Their interaction with the outside world is through specially designed headgear that reads their thoughts, allowing them to 'live' - if only through machines. Many Bloodless thirst to collect knowledge at any cost, hoarding it in a collective databank known as 'The Grid.' The sub-zero blue syrum that fills their veins has been known to gradually cause insanity in a small percentage of the population.

Wolves - The Wolves are an elite mercenary guild of starfighter, bomber, and mecha pilots. Accepting only the best into their ranks, they live life on the cutting edge. Striving for the latest in wartime technology to make their chariots of war faster, stronger, more powerful, the conflict is a natural place for them to earn a few extra credits. Though technically independent faction that sells its services to anyone for the right price, the Wolf's Den has been chartering more and more missions for its own volition lately.

The EYE - A splinter group of humans with psychic abilities, The EYE recruits its soldiers from a very early age. Latent psychic ability, particularly psychokinises, develops strongest around puberty. The EYE is the result of a privately funded research project into the paranormal (and is still privately funded by an unknown beneficiary). Fighting from a different perspective, they are known to dominate a battlefield through thought control and physical prowess. The weakest link in any vehicle is the pilot, is it not?