Denmark gets alot of permanent modifiers to mercs (reduced cost, extra discipline, reduced upkeep) as well as access to a bunch of unique merc companies.

引用自 BlueBangkok：

Any tips how to make good use out of mercenary ideas and playing with mercs? And which countries would be the best for it?

"Playing with mercs" is something I do with literally every nation. No starting country, even the strongest, has a large enough manpower pool to sustain the many early wars that I will fight. Mercenaries, especially the almost-correctly named Free Company, are thus a must in the early game.

By sometime in the mid-game, though, I phase them out. This process happens sooner for strong starting nations and later for weak ones, but eventually I'll have such a large manpower pool that it grows faster than I deplete it. Mercs become a net negative at that point because:

1) They generally cost more
2) You can't control their army composition. This can be somewhat compensated for by attaching additional infantry or artillery, but there's nothing you can do about a merc company with too much cavalry.
3) Hiring them lowers army professionalism for most nations (unless you take the Merc idea group).

And I don't take the Merc idea group. I'd much rather spend all of those points on a permanent military tech lead and lots of generals, which will max out my army professionalism fast. As I always say, the combo of a tech lead + great generals + max army professionalism is stronger than any single military group--and that includes the Mercenary idea group. I pretty much never take a military group before sixth for this very reason, if I ever do.

Now, of course you can play it in a meme-run to shake things up for fun--and you'll want to take it early in such a run. Mercs are strongest early, so take it early. But, when you do, expect your army to be significantly weaker than it would have been otherwise.

If you're doing a "serious" fun-run with mercenaries, understand that you're going to be limited in who you're playing as. Denmark into Scandinavia is great, and both Switzerland and Venice are good, if we're talking Europe. Anywhere else is...fairly limited...to the point that you're basically relying on the sheer power of your economy to compensate. Meaning Oirat into Mongol Empire, the Jurchens into Manchu and Qing, maybe Ming itself.

More than ever, however, you need to step up your diplomacy game so that you never fight more than absolutely have to, and you use your mercenaries to swamp your single one or two enemies all at once. Your own professional troops are to be kept strictly for sieging, and not for fighting.

引用自 Marquoz：

Now, of course you can play it in a meme-run to shake things up for fun--and you'll want to take it early in such a run. Mercs are strongest early, so take it early. But, when you do, expect your army to be significantly weaker than it would have been otherwise.

Yeah, I know it's not optimal strategy. I do want to spice things up with some fun meme-y merc run (mercs + merc ideas), but I also want to have it somewhat optimal - e.g. not lose. I want to use mercs predominantly, that's what I meant - not the free company / independent army that I use literally every single early game to get started, but a dedicated merc game from start to finish.

Denmark, like RCMidas suggested, sounds fun. I will take merc ideas as 1st or 2nd, and try to pump out arty stacks to cover for them.

I'd say Switzerland is fun for a merc run. I made an oriented merc run for its achievement.
It's pretty powerful for any nation as mentioned. I still do use them mid-late game to siege forts a lot faster or as decoy if the opponent is strong.
The national idea isn't good enough to be a constant pick. I'd say that if you aim for achievements that don't necessitate a long run, it can be an option as the strongest national ideas won't make much of a difference.

If you want to go all in, you can start as Denmark, form Switzerland and then form Scandinavia afterwards.
I believe there are a couple more tags you can snuggle in for more merc discipline, but those were the main ones.

Eh, if we're talking tag-switching, then it's really only forming the Two Sicilies (Neapolitan or Sicilian culture) for an extra +5% Mercenary Discipline. Everything else is too specific or locks you out of Scandinavia (eg. through becoming Mughals, Italy, or Byzantium).