The Mach 1 needs an RM version to compete with the Challenger and Camaro. Likewise, the new Mustang needs an RM as well, since there will now be a new Challenger RM.
This, makes a lot of sense given that both competitors (old and new) already have them, could create some interesting new series online too, one for the classic and one for the modern versions.
Same goes for the Dodge Viper (preferably the '02 GTS as there's only one Premium version) as an alternative to both Corvettes and a Premium alternative to the Standard Oreca Viper.
There are plenty of other possibilities, being the only Premium FF 5-door familycar, the Mazda Atenza Sport 25Z could become a
touring car, although since the DLC already packs a gazillion Japanese cars (and one American car as well as the one Japanese and 11 NASCAR cars we've got for free) I'll try to focus on some European candidates instead.
Both Fiats 500 (could become modern and classic Abarth replicas), a rally version of the Lancia Delta Integrale would be nice and ofcourse obvious candidates like the BMW M3, Aston DB9 (would double the amount of Premium Astons

), etc.
If they'd made only one RM Ferrari I wouldn't go for the obvious 'Enzo turned into FXX' but I'd rather see a 512BB LM replica.
And what about the Lotus Esprit?
Strangely enough the Clio V6 could become an RM quite easy I guess, they'd only have to use the one already available in Prologue....