So I think I am mutating to a "old system" player with Warhammer Fantasy and Horus Heresy (which is technically an improved version of the 7th Edition 40k) - but I like it that way!
You're not alone there, I have stuck to the old systems I used to play back when they were supported (7th-8th Edition Fantasy and 5th-6th Edition 40K) because of a combination of not being hugely fond of 7th and 8th 40K and 1st Edition AoS and simply not being bothered to get into the more appealing 9th Edition 40K and 2nd Edition AoS because of the unnecessary 'compulsory' scenario books and the time being too short between edition updates.
I will build a cool ScarVet on Cold one from one of the Cold one Knights from my boxes (I think there are some Bits on the Sprue for the Carnosaur to make one), and hopefully I can test the "Cowboy" on Saturday against some High Elves! Hopefully he, a Skink Priest with Lore of the Beasts, 20 Saurus with Shields and Handweapons, 15 Temple Guard and a Stegadon will hold their ground!
Sounds like a solid starter list, all those Strength 4 and up attacks will make combat pretty easy for you against most Toughness 3 Elves, and the Steg will bring the thing Elves hate most - Impact Hits! Though beware of Swordmasters and White Lions, their ability to strike at their standard Initiative with Great Weapons is nasty, so you'll need to make use of your Bolt Thrower and spells like
The Curse of Anraheir and
The Amber Spear to soften them up a bit before engaging, preferably with your Impact Hit-dealing Steg!
The Curse of Anraheir is also a great way to bypass a particularly notorious Magic Item of theirs, the Banner of the World Dragon - normally that banner gives the High Elf unit in question a 2+ Ward against magical attacks, effectively rendering them immune to spells, but
The Curse of Anraheir forces the unit to take Dangerous Terrain tests if it moves. Because Dangerous Terrain tests are not Magical Attacks, the Banner is useless against that spell!
I will take a look at the Beastmen Book! Sounds awesome, especially since the "official" army book for them seems a bit lacking. And as much as I like to play the game for fun, I don't like being on the receiving end of the heavy blows all the time! :-D
Certainly the 7th Edition Beastman book saw a drop in power level when 8th came about, particularly when other armies received their 8th books and became even stronger, so I aim to give our Beasty Boys a similar boost to level the playing field! I don't know if you ever managed to play them with their 6th Edition book when you first started? Because if so you're in for a treat, because a good few of the rules in that book, that were ditched or nerfed in the 7th one, are making a comeback!
My Ogres are currently getting new bases, I have taken them off their old Squares and put them on the round bases, but never played Age of Sigmar with them. So now I am 3d printing Square bases and glue them back on familiar ground

And hell, they look just awesome on a Square base!
You didn't keep the old square bases you took them off from? Never mind, 3d-Printed ones will work equally as well.
I do agree they certainly look more the part on squares! I've always innately viewed round bases as '40K bases', so to see Fantasy models on rounds when used in AoS seems jarring! Square bases certainly look great when the models are formed into units and the bases become so pleasingly flush with one another.