Just as the title says, unite the Celtiberian tribes. Likely against Rome or Carthage. I don't exactly expect this Union/Confederation to last very long but if you can make it last, that's even better.
 
Had Quintus Servilius Caepio not managed to bribe the three ambassadors he found to kill celtiberian leader Viriatus back in 140 BCE, we'd probably see the celtiberian lusitanian resistance against roman rule not dwindling.
Fast forward a few decades, and an united (or at least semi-united) Celtiberia may become a competent military power and one good enough to face off against the romans.
 
Butterflying Carthage being in Iberia might do it. Rome expanded there as a consequence of Carthaginian influence. Rome was not overtly imperialistic until after the Punic Wars. Before that, they were trying to establish security from invading tribes and neighboring states. You could prevent Rome from looking to the area long enough for a Celtic kingdom to form.
 
While it would be more of a mixture of Roman and Celtiberian elements perhaps a surviving Quintus Sertorius could build a state that could last a little while?
 
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