Disney is insane if they do not add a massive viewing area (at least for 25k guests,) either sitting, or standing (10k if seating, and to clarify around 25k for the surrounding waterfront areas for standing.
Animal Kingdom pulled in ~10,402,000 guests in 2014 according to the published reports (I know, they don't get their data from Disney, but let's pretend that they are reasonably accurate), so if the park is open 365 days of the year, that's ~28,498 guests per day on average. Absolutely no need to build a venue to hold 25K people. Not everyone will have interest in seeing this show and not everyone will stay at the park that long. I think it's generous to say that 50% of the total daily guests would have interest in seeing the show and would stick around at the park late enough to see the show and given how Disney routinely has two performances of popular nightly shows, you can split that figure in half again. As such, a reasonable capacity for the show would be closer to 7,000 people.
By comparison, California Adventure pulled in ~8,769,000 guests in 2014 (~24,025 guests per day on average). World of Color runs two performances most nights and from all the figures I can see, their venue for World of Color has a capacity of 4,000 people. So, with two shows a night, they can accommodate approximately 1/3 of the guests that visit the park on average each day. Using that figure, the venue at Animal Kingdom would only need to be large enough to hold ~4,800 per show (with two shows).
With Pandora, night safaris, and a bevy of other things, I'd argue that there is plenty to do at Animal Kingdom that is unique enough at night (as compared to doing them in the day) that spreading the crowds out won't be that big of an issue.
Will you need a FastPass to see River of Lights? Almost definitely, but hopefully they can do at AK what they do at DCA and make that FastPass
NOT count against your FastPass allotment.