Gunships move more than an order of magnitude faster than starships and should get a distinct icon.
Fleets containing nothing but explorers should also get their own icon so that players don't have to check every jumpfleet that enters their territory to see if it's an attack fleet.
The cutoffs should be:
Explorer propulsion: New explorer icon
Jump propulsion: Existing jumpship icon
>=3LY: New gunship/ramjet gunship icons
<3LY: Existing starship/ramjet icons
Perhaps fleets containing ships capable of landing should also get some sort of visual distinction.
Revised to reflect Era 3 changes.
I support adding the Era 4 tag to this: having different icons for each of these types of fleet compositions:
1. Jumpships (already done - triangle)
2. Starships (as above - rectangular)
3. Ramjets (as above - rounded blob)
4. Explorers (NEW - circular? Like this: o)
5. Transports (NEW - multiple triangles? Like this: <<)
The assigning of icons for each fleet type could follow the same rules for how jumpship/starship fleets are represented: If the fleet is composed of only that ship type: use its default icon as above. If another type is present: its icon should "override" the other. And they should override each other in this order:
Transport > Starship == Ramjet > Jumpship > Explorer
If different empire colors are to be implemented, I'd also support different colors for each fleet type: green = player, yellow = neutral, red = enemy, blue = allied (perhaps for later Eras)