First of all, before I cause any missunderstanding, this makes me happy if anything. But here are some things I fear:
(Nothing personal against the show makers, the writters or the animators, this is based on how the cartoon world is nowadays)
Given the simplicity of most of today's reboots, I can't help but imagine this reboot might be nothing but a lighter version of what it was. By this I mean it lacking a lot of characters such as Dr. Scratchansniff, Plotz, and even many segments/mini-shows. About that last thing, I wouldn't personally care if they don't reboot Godfeathers, Katie Kaboom, Chicken Boo, Mindy and Buttons or Hip Hippos because I never really enjoyed those, but if they don't reboot Slappy Squirrel and/or Pinky and the Brain it would mean they are probably not thinking great for the show (I liked Rita and Runt too and would like them to come back as well but I think it's not objectively as relevant as Slappy or Pinky&Brain).
Main reason I believe this is because I really doubt they will go find a bunch of voice actors and writters to make so many segments like the original show had, considering the simplicity of reboots in general.
In summary I honestly believe odds are they will only reboot the Warner Siblings with a very light visual style like the Powerpuff Girls reboot or Wabbit, and with a softer comedy as well. The voices will probably be different too.
Maybe most of you will despise me for what I'm gonna say here but I'm more hyped about seeing Slappy and Skippy back rather than the Warner siblings. Mainly because the Warner siblings had a lot of episodes (way more than Slappy of course since this one was a "side segment") and even though they still have the potential for more, Slappy had (in my opinion) the same potential as them (if not more) and it only had very few episodes.
I honestly enjoyed Slappy more than the Warner siblings, and if they do a reboot without her, it would be much more irrelevant to me.
To put it in simpler words, Yakko, Wakko and Dot had like, I don't know, 80 episodes? when Slappy had about 22. I'm not saying Slappy was way and objectively better than the Warner siblings, but for me, in all honesty, it was as good or slightly better, and it would hurt to see more episodes of the one that had about 80 and not a single one more of the one that had 22 and I liked more.
But yeah, I'm a minority after all. Maybe only 5% or less of the people that remember Animaniacs want Slappy back as much as I do. But nonetheless, it would hurt to throw all the segments off the window an stick only with the warner siblings. If you ask me, it would mean that the reboot has weak roots and aims for a lighter, less relevant production with lower budget and inspiration.