Let's draw upon an idea from another game.
The idea is: Put a hard cap on a player's attack fleet size, and defense fleet size. This means that when you fleet is destroyed, it is only a small percentage of your actual ships available, and so the fleet can easily be replaced!
Hmmm.
Perhaps the attack/defense fleet size could be limited by your population.
Or by the planets you control (too many ships in orbit means that planetary fuel runs-out)?
And the most complicated possibility would be planets supplying fuel to other planets, thus increasing the maximum attack/defense fleet size.
But basically, the premise is that I feel:
1) In an RTS, players want to relax. They do not want to deal with fleet-loss.
2) If fleets are easily replaced, then the stress is lowered.
3) If the fleet size is capped, this adds more strategy to fleet-formation and ship-design.
4) Capping the maximum number of fleets being sent out on attack will also limit the time that the player spends in-game, so that they do not get bored of the game, yet instead have to login consistently in order to be there when their attack fleets return home.
5) Attack fleets can raid… So changing your formation gives you an increased chance to 'evade' defenders and pick-up loot.
6) Since the game is based-upon resource production with respect to time (instead of large stockpiles), the 'raiding' system will have to be carefully designed.
7) If your attack fleet or defense fleet is destroyed - no problemo! It is automatically replaced (when the enemy leaves), until you run out of ships. E.g. A raiding fleet can attempt to evade defenders, or else form a "blockade" that can only be broken when the player comes online or the attack fleet departs (and the defense fleet is automatically replenished).
I apologize for not being able to find the name of the game. Fleet-size caps are uncommon in browser-based games, yet caps on number of raid-fleets is common.