By tier, I mean different starbases that become available in succesion on the Defense tree.
Yes, I believe so.
I also am interested on rendering new looking starbases with new or enhanced abilities depending on the specialization of the starbase. I'm thinking that each race would have a support and physical type of starbases to go with the basic version.
Graphical work requires a bit more work as you have to make new mesh and texture files for each Starbase, but if you know how to do such things it is quite possible. Even if you don't with enough determination you can do it. Abilities take some time to figure out, but it's not so hard once you get the hang of it. Depending on the desired effect you will at least need to make a new ability file and possibly a buff file if it has multiple effects.
Also, one problem I know I am going to need to fix first is creating new entity files. Whenever I rename a current file, it defalts to a txt document and won't give me an option to save as an entity.
I usually just copy and paste a file I want to use as a base to another folder and then change the name using window's rename function. Sparda's method should also work.
Tougher stuff could also be handled by my brother, if not by me, he has taken classes on designing games.
Everygame is different but yes he could be helpful, especially if he knows how to model. 
Anyways, of the top of my head files that you would need to create.
NewStarbase file
Starbase upgrade files to go with the new starbase (if you are using the same upgrades skip)
New Starbase construction ability
New Starbase abilities (if applicable)
Research for starbase and abilities
Any new meshes or textures
Files to modify
Player(Race).entity
Constructor ship of the race
entity.manifest file
String
I'll probably add more as I think of them, but hopefully that is a decent start. Try to do it in managable chunks, like get constructor ship to make a new starbase, then add the research, then start making it look different etc. Feel free to ask more questions.