Could a city in Iberia have been chosen instead of Ravenna? If so, that might be a start, and then in some way avoid any incursions of the tribes, so that peninsula is kept securely Roman, perhaps by only employing individual mercenaries in those lands.
It's unlikely for several reasons.
1) Moving capitals or having imperial residences wasn't made to protect the court (even if it was a factor) from incursions but for having administration and army being able to react quicker to them. Milan, Ravenna (that didn't host the court on a regular basis before Honorius, at the moment where Barbarians began to settle Hispania), Treves, etc. were less capitals than residences for an itinerent imperial court that needed to be able to react to threats.
2) Having an imperial residence in Spain would mean that emperors would have considered Gaul as lost, as if they couldn't move their court there to react against Rhine incursions.
If they would have chosen to abandon the province, it's far more likely that they would have tried to keep Italy as a "national redoubt" : still wealthy, core of imperial power and close enough from ERE to be possibly helped.
3) Look at a map of [URL="http://pages.uoregon.edu/mapplace/EU/EU19%20-%20Italy/Maps/EU19_65.jpg] Great Invasions[/URL]. Spain is the (wealthy) dead-end of european migrations and invasion of barbarian peoples (and I spare you the III and IV raids). Lack of a fleet, except for Vandals, means that whole peoples were stucks in the peninsula : Vandals, Alains, Goths, Suebi, etc.
It would be really hard to deal with all of these, and if they aren't stopped at the Rhine, they surely gonna end there (rather than Italy, even if it was a good target itself).
4) Mercenaries in the late Empire are almost always (and almost here is for convering me) barbarians : Huns, Goths, etc. not always that much distinguished from laeti or foederati and engaged directly by generals.
Using them as a main protection of a province would have been hugely dangerous and would have : at best a Barbarian (as Sitilicho or Odoacer) ends by being a ruler de-facto if not de jur, at worst you set the cats among the pigeons and only helped barbarian incursions or settlements, both meaning an at least partial loss of the province virtually without opposition.