It's not very likely that the Allies, when truncating Hungary so drastically in other respects in the Treaty of Trianon would make an exception for Slovakia (they might conceivably have allowed the Hungarians to keep the Magyar-majority southern strip of Slovakia but even that is unlikely). Some Hungarian democrats like Oszkár Jászi proposed a federal Hungary along Swiss lines; but the problem is that before the First World War (when this might have satisfied the non-Magyar peoples of Hungary) this had no chance of being adopted, and after the War it would satisfy neither the non-Magyar nationalities nor the Allies.
The only way I can see Slovakia as part of Hungary after the Allies won the War would be for Slovakia to become part of a surviving Hungarian Republic of Councils (i.e., soviets) in a TL where the Bolsheviks are much stronger in 1919 than in OTL, and the Red Army is able to come to Béla Kun's rescue.