I'd say they need to introduce a new class of ship.
It would be part support ship, part combat. Each race would have different variations of them.
They would be "Sub Capital Ships"
Take a beating like capitals, yet be a lot more spammable and cost around 30 fleet supply per. Each race would have 5+ different types with different fleet capabilities to really mix things up.
More variables + more paths to victory = good game design, imo.