This is a good topic. Some comments:

Uncommon indeps:
There are some uncommon indy forces that show up occasionally (and you will usually get a clue that they might be dangerous when you see some uncommon troops in the province, like living statues). In these cases the most prudent course of action is to probe first with a scout before committing your army.

Two exceptions to the above:
-Independent commanders some times get random magic items, from time to time, fortune results in one of them getting a soul contract. If you start seeing devils in an independent province, it is a sign of this. This province should be taken soonest, because the longer you go the more devils you will have to face.

-Ever since the times of Dominions PPP, there are 3 air mages which have an habit of showing in provinces that just report as light infantry & militia defenders (can anyone confirm that this is still the case in Dom3? it's been a while). These 3 air mages with their armor negating spells have been the bane of countless expansion armies & pretenders. So, careful about sending your precious awake pretender against such targets.

Finally, always keep your eyes open for indy populations that complement your nation, and make sure you claim those provinces if they show nearby. Getting some Garnet amazons for example, can be huge for your game.

Underwater expansion:
The defining factor in underwater expansion is poison: many troops carry poisonous weapons, or even a poisoning barbed armour. You have to be able to deal with this to avoid suffering heavy loses when expanding underwater.

Thus, expansion pretenders without at least some poison resistance or regeneration are not a good idea, because even if they have a lot of hit points they will stack up afflictions from so many damage ticks.

When it comes to troops, you might not want to use either heavily armoured elite troops with low defense in expansion, because being hit every turn with a poisoning spear hurts. You particularly do not want to use troops with pincer/punch (low reach attacks) vs triton troopers, because you will get poisoned by their armour every time you hit them. Surprisingly, you do not get poisoned if you trample, running them over. I guess Dominions is weird at times.

The psychology of expansion:
Guides often tell you that you have to get X provinces by Y turn, or you are a bad player. This is not written on stone, it really depends on the game settings.

Obviously, you do not want to be the guy surrounded by neighbours who are twice your size, because then you are just food.

But hyperexpanding in a game with graphics on & diplomacy will just paint a big target on your chest. Really, the most basic diplomacy move is: "look how big & aggressive this player is, let's gang up on him before he attacks us"

Thus, never forget the game settings when planning your expansion: Games with no diplo &/or no graphs favour hyperexpansion strategies. In games with diplomacy &/or graphs enabled, drawing too much attention might actually work against you.