1 and 2 can be accomplished simply by limiting base purchases to one per system per turn. I don't have supply in my rules, so I don't have much to say about 3. I would like it if the system (or possibly  the system including optional rules) would generate convoy raids as an emergent effect.

As I've wandered around this design space, I've sometimes thought of doing a 4X game that relies primarily on the tile-laying mechanism of the board game Carcassonne -- hence, Starcassonne. There's a bit of that going on here.

Eliminating downtime is a good thing in a face-to-face game. In a play-by-email game, it's probably more important to get as much done as possible before turning things over to the next player. I'm not sure which is better for a campaign game like this. Maybe doing a lot of simultaneous decision making (as in the plotted movement in Starmada) is good.

Being able to stop the game and record the state whenever a session happens to end is important for a campaign game. That argues for doing everything on paper, but that would be (to me) very unappealing if the game was being used as a board game without Starmada integration. If VBAM is out there (and being revised) for those who want a deeply detailed campaign, maybe SovStars should focus on the simpler end?

I guess it's really the ability to have multiple bases in the same system that provides multi-hit bases. In other words, where you have bases at three strengths, I represent the same thing by one, two, or three bases in the system.

Note the following from the "Light Bookkeeping" section of OoM:

I recommend it. Carcassonne is one of my favorite "gateway games", used to introduce serious gaming to people who know of nothing but Monopoly and Scrabble.

Noted.

Maintenance rules can be good to prevent players from "turtling up", staying in place and building larger and larger navies.

Supply is only important if cutting off enemy supplies is a reasonable tactic. If players build empires without long tendrils and never launch expeditions deep into enemy territory, supply may be needless bookkeeping. On the other hand, if the rules can be engineered so that such missions are a good idea, life might become more interesting...

(I think the current OoM rules, which do not include supply, do encourage players to rush out and grab valuable choke points as the galaxy is discovered. Later in the game, forces are concentrated at such points, but some are also used for exploring the remaining territory, which can sometimes open new routes of attack.)

Assuming we do want such rules, we could simply require a continuous chain of friendly units back to a base (any base) to constitute supply. It might be reasonable to add a third type of unit, a convoy. This unit would have little if any combat ability, but would be much cheaper than a regular fleet (perhaps 2 or 3 for the price of a fleet).

Yeah, striking a balance between simplicity and richness it tricky. I love the game of Go but it's not big on story.

I would argue for giving them all the same number of battle dice and just changing the prices. One parameter is easier to balance than two. When resolving battle, counting is always faster and easier than adding up a bunch of numbers, even small ones. If these units are all just abstract agglomerations of Starmada ships, it's easy to say that each one is of the same fighting strength. If the strategic value of their abilities can be balanced, making them all the same cost has an appeal, too.

Can you say more about why you believe supply and maintenance are essential? Are they worth the bookkeeping?

What power do SDFs have that allows them to control systems where fleets cannot? Is it that they contain surface armies? If so, does it make more sense to separate the armies from the ships, rather than armies-and-control-oriented-ships from non-control-oriented-ships?

...but you have to do that every turn. This slows things down. Are there better ways to prevent turtling?

One possible solution could be to let the players do their turns simultaneously without any specific order. This may sound a little bit chaotic, but I know with mature players it is not. If there is a disagreement between 2 players (eg two players refuses to make all or part of their navies before one of his enemies does it - eg. because they want to see how the enemies places their ships) a special player is the judge who decides which player has to begin to move.

This could be combined with some "hour glass" mechanic, where there is a limited amount of real time is available to move his ships.

While simulatenous strategic movement is AFAIK not the standard (it works with mature players very good - for our gaming group I invented a 4x game with such a movement mechanic in order to cut boring waiting time - but this was years ago), working against the clock is indeed. For example the adorable Space Hulk uses a clock for the marine player.

Another mechanic could be the following. Everybody moves his navies simulataneously in eg a 4 player game. After the 1st and 2nd players are finished with their move, they say so. After the 3rd player is finished he says so AND he turns an hour glass which initializes the time limit for the 4th player.

I know this is totally strange mechanic for the average wargamer but OTOH its cuts the waiting between the turns considerably. Of course there is an additional drawback: movement of navies AND combatphases has to be divided in 2 seperate phases and no pinning of fleets is possible. (because every movement is "simultaneous")

Why?

Think of SDFs (or armies, or corps, or whatever) as providing the necessary infrastructure to actually exploit a system's resources. Fleets can "control" in the sense that they can fight off potential rivals, but you still need the ground-pounders.

I would REALLY prefer to avoid separate "space battle" and "planet battle" steps.

Another reason I like the idea of a maintenance cost:

If players know their empires can only support X number of units, it makes the decision on what to build that much more important. For example, building another base not only costs more money, it also takes away the ability to support an additional fleet or army.

A thought: Maintenance cost is doubled for any units not in the same sector as, or in a sector adjacent to, a friendly base.

Hear, hear! (This may change if many optional rules are used, especially when incorporating with Starmada, but a stand-alone board game should at worst require pencils for scoring.)

I don't think that's directly the problem. The problem is when people choose to amass ships rather than exploring or fighting and the game becomes a stalemate.

Some kind of game "clock" would work wonders to prevent stalemates. The simplest clock is what older versions of Starmada scenario uses: the game ends after N turns. Games like Mastermind, Carcassonne, Agricola, and (to a lesser extent) Scrabble use this mechanism. More interesting is a clock that can be sped up or slowed down (but not stopped) by players' actions, as in Lost Cities, Tigris & Euphrates, Container, Ticket to Ride and Puerto Rico.

More discussion of this issue can be found here:

http://www.boardgamegeek.com/geeklist/32208/temporal-shifts-timing-mechanisms

A similar limit is present in the current maintenance costs: since each system generates $3 per turn, you can only have three times as many units as you have systems. That leaves very few units for expeditionary forces. Also, since movement is so slow, you can't hope to coordinate a bunch of units for a surprise attack; the enemy will see them marching to the front.

You could theoretically increase your income with the bonus for bases in shared systems, but we found such systems too hard to build. For an alliance of two players to gain anything from this, they would need (combined) at least three bases in the system. (With two, nobody owns the system.) This costs $15 plus the time to move armies there and then build the bases, as well as the maintenance for the units involved. The player who contributes one base gets only $2 per turn; after $1 to maintain the base, that means this won't pay off for 5 turns. The player who contributes two bases gets only $1 additional per turn -- only enough to maintain the second base!