Unfortunately, I've been sick most of the past week, so this analysis may not be quite as detailed as I'd hoped. Nevertheless, we move forward!
Also, a special shout-out to Scryfall (https://scryfall.com/), which has been immensely helpful in putting this analysis together.
Final numbers for Amass, +1/+1 counters, and Proliferate
Per draft of WAR, you can expect 32.5 Amass cards to be opened; 11.7 of these are mono blue, 9.3 mono black, 8.0 mono red, and the rest multicolored. So if you have an opponent in two of these colors, you should expect them to likely have several Amass cards. But if they're in only one of those colors, they'll probably only have a few. It's also good to note that almost all of these cards are Amass 1 or Amass 2.
In terms of non-amass cards that produce +1/+1 counters, you can expect 12.8 of those to be opened per draft in White, 8.4 in Blue, 0.9 in Black, 6.0 in Red, and 12.1 in Green. So in the proliferate colors, there's lots of cards that are going to enable proliferation (note that blue also gets to count the Amass cards as noted above, so they're actually in first here). On the other hand, this is not so many cards that you can expect to have just tons of different permanents to proliferate onto at any point - one target will happen, two will be common, and you'll be quite happy to manage getting three.
Speaking of Proliferate, it ends up on 5.7 cards per draft in both White and Blue, and 8.4 in Green. So, don't expect to be able to build a deck around proliferating over and over again - unless you get one of the cards which singlehandedly pumps trigger after trigger out, you're more likely to get one, or maybe two over the course of a game, even in these colors.
Is spellslinger a real strategy?
As is so often the case in these sets, Blue/Red's theme seems to be "spellslinger", i.e. it wants you to play lots of instants and sorceries. Slightly confusingly, in this set in particular, some of the cards in this direction point you towards those particular types, but some care about noncreature spells more generally. And I expect most decks in this format to have a few noncreatures which aren't in these types (mainly planeswalkers, though there's some playable enchantment-based removal as well).
So what are the numbers? Well, on the non-creature side, there's 5.1 monored, 4.2 monoblue, and .9 hybrit Izzet cards per draft. And on the Instant/Sorcery side, we're at 4.4 blue, 1.5 red, and 1.3 Izzet gold cards. Overall, if you're completely alone in your lane, you might be able to scrape together a deck based around these.... but I wouldn't really bank on it. I think the biggest way to get into this deck is to open a good rare that's on theme and then pick up another couple early - but don't be trying to get payoffs later, it's just not likely enough to happen.
Of note, the red cards here also work with the red-white (even less supported) subtheme of pumping your own stuff - I don't think both of those decks can exist at the same table, though obviously you can build decks in these colors that don't exactly follow those themes.
Mana Fixing, or lack thereof
There are 10.4 pieces of mana fixing per draft which are colorless (i.e., lands or artifacts); you get access to an additional 6.6 if you are base green. This is actually a reasonable amount of fixing... but it's a LOT less than we've seen in the last couple of sets set on Ravnica (or actually, any of the Ravnica sets). So five color decks will be nigh impossible to make... three color decks are even going to be very ambitious. Especially if you aren't green, you'd need basically all the fixing at the table. Plus, since most of the fixing isn't in lands, you would end up with like half your spells just being dedicated to fixing, and I just don't see the payoff being worth it. (Sorry, Niv-Mizzet).
Having said that, splashing seems very plausible. It's definitely not to the point where you would say that splashing some spells from a third color is free, by any stretch - you still have to work a bit to get your mana to get there, like normal - but if you have a reason, you should be able to find something to get you there most of the time (provided it's not like, halfway through pack three already or something).
How big are the creatures in the format? Obviously it's a little bit tough to tell just by looking at a list of cards, since there are questions of playability, plus a lot of +1/+1 counters running around and affecting the sizing.
But if we look at everything, just on the base stats, then in terms of power, there's a massive hump at 2 power. There are nearly as many creatures with exactly 2 power (58.1 per draft) as there are with greater than 2 power. Moving from 3 power (28.2 creatures per draft) to 4 (21.1 creatures per draft), there's not nearly as big of a drop off. Per normal, not many creature get to the 5+ power range (12.4), so don't be super surprised if your opponent has one of those, but it won't be often they have multiples.
On the Toughness side of the equation, things are more spread out. 31.4 creature per draft have 1 toughness, so most of your opponents will have a target for your ping effect to hit (although in many of these cases, you would need to time it precisely, as a few of these creatures grow from an ETB counter, and some others are unplayable... so be ready to sideboard around this situation one way or another, which ia fairly common problem if we're honest). 50.2 creatures opened have 2 toughness, and 43.4 have 3. This is the point where the biggest drop-off is, with only 20.4 creatures per draft having 4 toughness, with an additional 16.5 at 5 toughness, and 7.9 at 6.
In the hopes of finding a "magic toughness" or sizing in general, it's also important to look at the toughness-based removal the set provides (damage or -N toughness). 6.4 such cards per draft punish 1 toughness, 8.2 on 2 toughness, 4.6 on 3 toughness, 5.1 take care of creatures with 4 toughness, and 2.3 (1 common) deal with creatures having 5 toughness or less.
Based on looking at this, I doubt that there really well be any "magic size" for creatures in the set, though I guess that most things with 5 toughness will be fairly hard to take out using a single card, especially if that card isn't one of the few premium removal spells in white or black that don't care about size at all (or the fight-like spells in green which are pretty close)
Overall, The biggest thing about this set is that it looks much closer to a 'normal set' to me than we've had in a while. Well, except for having a couple of planeswalkers per deck, which is, I guess, a pretty significant difference. But the fixing numbers, creature sizing, and for the most part lack of cohesive on-rails plan for each color combination makes things mostly more block-and-tackle. Or more, uh... I feel like there should be a better metaphor which doesn't draw a parallel to a sport which is virtually exclusively played in a single country. Anyway, I digress.
Take good cards, probably don't splash, realize your opponent will have a couple planeswalkers, but also realize they won't be the be-all and end-all. Try to have board presence. And most of all, have fun! It's a new set, that's what they're for.
Hopefully I'll have time to get a moxiously early pick-order list generated before the end of the week, with some notes about specific cards, but we'll see...