really excited for this new book

The Fellowship won't be a Battle Company, those are all about basic troops becoming heroes (although you could use use a Legolas mini to lead a wood elf company, or the hobbits as the leaders for a Shire warband). Rohan is likely to be good fun, plenty of room for promotions (warriors become Riders of Rohan, Royal Guard, one day perhaps Sons of Eorl) and some nice hero figures to potentially use as your company's leaders. Isengard should have a decent selection as well.

It's bound to be something along the lines of Scouts > Warriors > Berserkers, where they may fit in Mauhur's Marauders and Ferals in some fashion (don't think those existed in the original release). The current fan supplement gives the player a choice to have a promoted scout turn into either a Marauder or a Warrior; the Marauders can then become Ferals while Warriors become Berserkers. Given that there are no models for Marauders, I can imagine those being left out - the progression might simply be Scout > Warrior > Feral > Berserker. Worth noting figures will only have a chance to be promoted after a game or two typically (depending on how well they do), and then roll a D6 to either not promote (roll of 1-3), be promoted (4-5) or become a hero (6), the latter of which still using the same model. In other words, Berserkers are likely some way off - especially since not all members of the company will survive that long!