Been on a bit of an EU4 binge lately, and I recently just realized that Gaeldom, which starts as part of Scotland, has some pretty great national ideas. Among them are +15% shock damage, which is just fantastic, +15% army morale, +10% siege ability, increased attrition for enemies, and some manpower modifiers. It's not quite on the level of Prussia, but it wasn't even all that hard to take over the British Isles pretty quickly. Just invaded Sweden now, and even with their national ideas they can't keep up with mighty Gaeldom.
Worth a try if you feel like play and underdog that turns into a military powerhouse.