Whether or not to limit player choice in party design

Weekly updates:
This week I played a game with an old friend Devin.  His feedback was really positive, and he seemed to get the rules quickly.  He especially liked the new animated creatures (which I put back in).  I don't know whether this result is good or bad at this point, since I'm not sure if I should be improving Illeria or verifying it works.  Maybe it just is.  

I've also worked on summoned creatures.  In my game last week, Nick thought they were too powerful, because they could pop out and kill an enemy out of the blue.  His suggested weakening the creatures.  I have been toying with solutions, and I think I'll make it that the creatures can't act when first summoned.  That way, they can remain strong, but enemies have a turn to respond.

For this week, I wanted to discuss whether and how to limit choices in creating a party.  In Illeria, you'll field a party of 5-ish characters.  I've debated whether players should be allowed to choose any characters, or if there should be limits.  This post is inspired by a great roundtable at Guerrilla Miniatures Gaming.  This is the third roundtables they've done, and they've all been pretty inspiring.  

When I started playing Warhammer 40K, you could spend at most 50% of your points on vehicles, and needed to spend at least 25% of your points on infantry.  In later editions, an army needed two to six units of troops, zero to three units of elites, etc. I found this obnoxious.  I understood why you'd want to prevent players from bringing just tanks, but I wished this could come about more organically; like, that an army would fare poorly without a few units of infantry.  

There are games that have done organic limits well, such as Magic the Gathering.  There are no rules preventing you from playing a five-color deck, its just that such a deck would be terrible.  The way Magic accomplishes this is that you need land cards to generate mana, and different spells require different kinds of mana.  So, if you are playing a one-color deck, every land generates the mana you need; if you are playing a five-color deck, you often find yourself having the wrong kinds of mana.  The other way I have seen organic limits involves synergies.  For example, in Slay the Spire, the benefit of every card depends on the deck.  For example, a card that triples an enemy's poison is amazing in a deck of poison cards, and useless in a deck without poison cards.  

40K also had another limit: each player could only take units from one army.  This made sense both thematically and from a gameplay perspective.  Thematically, each species was fighting for survival against other species; thus, it wouldn't make sense to have Orks fighting alongside Eldar.  From a gameplay perspective, choosing one meant accepting the army's pros and cons.  For example, the Imperial Guard had lots of tanks and guns, but the few psychers (space wizards) or melee units; if they could bring a couple Eldar psycher and units of Chaos berserkers, they'd be the best at everything, and this would rob the game of something.

So how to these apply to Illeria?  Should there be parties of summoners and parties of spellcasters, or should parties be able to have one of everything?

-Thematically, it is not clear why spellcasters and summoners wouldn't work together.  Every character is a mercenary in a party that is prospecting Illeria.  If anything, a party would want to have different kinds of characters to deal with different situations (the same way you'd never have a D&D party of all wizards).  
-At times I've played around with limited and varied resources.  In earlier editions I put limits on the number of characters that could act each turn (so that some characters would get skipped every turn).  I had rules that allowed you to activate extra characters if they were similar, e.g. multiple characters could make an archery attack with one action.  However, I eventually decided that I wanted every character to be able to act each turn, which eliminated this resource.  
-I have played around with synergies.  For example, the ability Aura of Magic that gave allies bonuses to spellcasting.  There are two problems with this, however.  First, while some synergies will arise naturally, most require a character to have an ability that boosts other characters (e.g. Aura of Magic gives allies a bonus; an ability that gave the character with the ability a boost wouldn't affect the other characters in the party).  I wanted most abilities to affect individuals, rather than be ``All characters in the party gain X."  Second, if I make synergies really powerful, then I feared battles will be won by the party that comes to the game with the best synergies, rather than by the person who plays the best (which, as I've said previously, I'm trying to avoid).
-I've never been a fan of arbitrary rules about who a player can bring, so I want to avoid that.

That said, I worry this will lead to a game where every party is a generalist party.  My brother-in-law Dave once had this commentary about a Star Wars RPG without limits.  He said that characters could take any power, so really every character should be a medic with particular powers, which made the game uninteresting.

I'm nearing 1,000 words, so I'll be brief.  I'm playing around with a thematic idea.  In Illeria, there are multiple artificer guilds that send prospectors into Illeria.  I'm thinking of dividing abilities into six archetypes, and saying that each guild specializes in two.  I might make it that in the campaign mode, parties occasionally find items allowing them to take something outside of the two, but that will make the spellcaster in a melee/archery party really special.  I'll also probably make that optional for pick-up games.