I'm a little bored with 4X games having a finite linear tech tree. There's no reason why a tech tree can't be infinite, or linear for that manner.
For instance, you could always add +1 or more to various attributes of whatever.
Have a look at how research is done for Capitialism or Caitialism II. Research is infinite! It woud be great to adapt a system like that for a 4X game.
Say you wanted to add to an attack value. You might choose +1 which would take 10 turns with current research rates or you could choose to go for +2 attack instead which would take 19 turns, or +3 which might take 27 turns. You get increasing rate of return the longer the projects.