Nice, I am going for Empire of Rus, then, Slav U and finally Russian empire, knocking off 2 achievements (Russkaya Pravda and Lech, Czech, and Rus ).
Which reform would be the best for expansion? I think I am going either enatic or agnatic for open succession, but not sure about the rest.
Best reforms depends on whether you intend to stay tribal forever or adopt feudalism. The main benefit of staying tribal is that you get to have a huge retinue fueled by prestige. Use Hunting Party which combines 100 Light Infantry with 50 Light Cavalry and has excellent synergy between the two unit types. It's only slightly weaker than a pure Light Cavalry nomad horde of equal size would be and much cheaper. It would be even better if you were West Slavic or South Slavic, but even for East Slavic it works the best.
If you want to be cheesy there is also the option of so-called "North Korea mode" (NK for short) where you personally hold all the counties and holding and ignore demesne limit. That actually works quite well as tribal because you get a penalty to gold income and levy size, but raid income, prestige gain, and retinue size are not affected so you can have an even bigger prestige retinue army. Most of the retinue cap comes from the basic hillfort, which will likely exist in most tribes already and if not is easy enough to build.
The downside of staying tribal is that you're stuck with Gavelkind ... unless you enable a better succession with your religious reform. So let's talk religious reform.
Nature
The big question here is whether you will have vassals or not. If you will the best choice is probably Proselytizing because it is the only nature which will make your vassals consistently convert their counties. Unyielding is also a viable choice, but vassals will only convert their provinces if they are zealous.
If you will go tribal NK, neither of those will do much for you though. You'll have to do all the proselytizing manually which is going to be a slow process and likely never complete. TAs a result you won't get much benefit from Unyielding and Proselytizing and Warmongering won't do much for you either because you have no vassals. Proselytizing does still make converting provinces faster, so it's not a complete waste, but you will get less out of it than normal. And you don't want Peaceful because it would get in the way of conquest. That leaves you with Dogmatic and Cosmopolitan and I'd probably pick the latter in most cases and Dogmatic if I wanted to get the high god synergy.
Leadership
This one is pretty simple:
- Temporal leadership is the default. It gives you full control over great holy war targets and your personal piety contributes to moral authority.
- Hierocratic is useful if you're playing with defensive pacts on. When the high priest declares a GHW it won't trigger the pacts against you because it's not you who is starting the war. But if you're playing with pacts off then it's inferior to Temporal, despite excommunications.
- Autonomous is for people who don't want to conquer the world and would rather get a stat bonus.
- Autocephalous is for the rare case where you have other independents of your religion and you want to help them stabilize their realm. This is extremely rare.
Doctrine
If you're staying tribal you'll need a better succession law. This applies to both NK and standard play. Your three options are Ancestor Veneration (works best in combination with Hierocratic leadership), Agnatic Clans, and Enatic Clans. Any of the three will work fine, so pick whichever you prefer.
The second doctrine doesn't matter all that much and you might as well go for flavor. Bloodthirsty Gods is fun and mixes well with tribalism. It'll eventually also give you more prestige for you tribal retinues which is nice. But you can really pick whichever you want.
If you want to go feudal you don't need a better succession law because you can already adopt Primogeniture. Meritocracy is nice because it allows you to select a favored child without having to use any sort of convoluted method, but it's not essential. You could also go for Ancestor Veneration and adopt Eldership, which allows you to pick anyone from your dynasty as heir.
The one thing to look out for are synergies. A few that I find noteworthy are:
- Daring + Sea-Bound gives you access to pirate traits which are the same as viking traits. Pretty fun, but Germanic pagans get that
with only 1 doctrine for free.
- Dogmatic + Divine Marriage + Temporal makes the current religious head the high god of your religion. Pick it if you want to be worshiped as god-emperor (or god-empress).
- Dogmatic + Religious Tax is not really a synergy, but it is a strong combination for feudal realms and especially merchant republics.
Though that bloodline tends to die off before I can grab it quite often... 
