Movable Defenses

I believe they are called ships, frigate, cruisers, capital ships, titans.

I believe one of the vasari factions has an ability to strip their planets down on the fly for resources. This could cause a large imbalance since you would have to make sure there are enough Tactical / Logistics slots open. You'd also have to account for where it would place all of those things in a new gravity well. I think an ability maybe "Rapture" that could teleport an advent starbase either to any chosen gravity well or at least move over one gravity well at a time would be a good alternate to the whole gravity well.

Who in the name of has ony one fleet? Even if my main fleet is out there bashing enemy ships I always keeps a few smaller fleets with defensieve capships around to guard my planets. And if an enemy cannot be held back by starbases plus tactical structures plus defensive fleet, well you'll have to freaking move all your fleet there ASAP to stop it. If you take down his fleet he'll be supectible to attacks, you might even be able to get more than just your planets back.

True I forgot what I normally do when the enemy is a bit more challenging than beta ai so you would have have more fleets.

Didnt think about how much space the other planet would need to have and if you enemy can moving it would be super hard to defeat him just wish there where more ways to retreat :/

Well I believe the Vasari loyalist have Starbases that are able to phase jump correct? And their minelayers kind of fit the description. 

Their Starbase can move around the gravity well, but not phase jump. 

OP's suggestion doesn't really work with defensive structures. It would change the way the game is played for the worse. 

Read what I said "Vasari Loyalist" the Vasari will be able to, I cannot remember if it was the rebels though and not the Loyalist.