Well, I was going to suggest the following things anyway, so why not here?
I would suggest a 2-type supply-system, fuel and supplies. [SEIV-like](I'll just do a summary, since I've worked for about 11 hours and its 4 am here...)
Fuel is used to operate FTL-engines. Without Fuel Interstellar-travel is impossible, in-game that would mean that the fleet that has run out of fuel is supposed to stay within the starsystem (or a particular point within the starlane if inter-starlane-combat will be possible, otherwise entering a starlane without enough fuel has to be prohibitet). A fleet-wide stock, I guess, is the best solution. Each ship consumes fuel according to its hull-size & engine. If a fleet is running out of fuel, you can split off great fuel-consumers and transfer all fuel to the splitted fleet in order to, for example, overdrive engine-speed to reach a certain point before an enemy fleet does. I guess I don't have to demonstrate the great tactical-options(killing of supply-ships (that store fuel&supplies... hmm, like the name says
), increasing range by adding huge supply-fleets). Solar-Sails / other techs that produce fuels while the fleet stays in a system are also an nice option.
Fuel-consumption is calculated by distance and speed (decreasing travel-speed saves fuel, since exponential more energy is needed to travel faster).
Enfols water, oygen, food, amunition, spar-parts, and so on.
Needed to supply crew & weapons & sub-light travel. Without supplies a fleet is an easy prey, since weapons/shields won't work (or just operates on a really low level). If a fleet runs out of supplies, crew begins to starve after the reservers are gone (would be a nice thing, just starve a fleet and then attack & capture the more or less defenseless fleet (depending on the ai-level [automated security-stations, and and and...] ).
Well, that are my 2 points.
(awww, Im a little bit tired, besides my english isn't that well, any corrections / feedback that is offtopic -> pm please