You don't need to make magic items to make monsters.
Cave bear, level 6: 20 AC, attack +10 for 1d8+Str damage
Dire bear, level 11: 25 AC, attack +15 for 2d8+Str damage
Does it therefore follow that the dire bear is a cave bear, possessed of a magic item called "the dire", which provides +5 attack and AC and +1d8 damage on attack, and which PCs must be prevented from looting at all costs?
I don't think anybody has ever seriously had that idea, and it really doesn't have to be any different when you have
John Thug, level 2: 16 AC, attack +4 for 1d8+Str damage, +1d6 flanking
John Thug, Guard of Corruptica, level 13: 25 AC, resist forced movement, attack +16 for 3d8+Str damage and -2 AC till EONT
- Your town guards aren't lesser enemies using advanced magic items that the PCs can loot and obtain an identical power boost from. They're better thought of as magically (or, you know, fiendishly, technologically, elementally, choose as appropriate) created enemies of an appropriate level, reskinned from existing monsters or made using the standard monster creation rules. And just like most other enemies, the PCs can't just directly access their power once they've fallen.
- If your PCs maintain an interest in adapting this "fake power" to work for them, adapt the existing loot parcel rules to provide them appropriate upgrades in-theme with whatever you've made, and consider adding some quest XP or a skill challenges to account for the effort they make in doing this.
- If your PCs maintain an interest in turning this "fake power" against its corrupt masters, in a classic move from the hero playbook, consider using the DMG2 companion rules to spice up some of the climactic fights.
I appreciate the fairly straightforward intent to strap John Thug into an Iron Man suit with a force trident, and some bullet points probably aren't going to be enough to address your concerns that Regdar the level 13 human fighter might be getting some unexpected upgrades. So here's a longer-form explanation about how the Iron Man suit and the force trident aren't "magic items" as commonly thought of, any more than anything about the dire bear was a "magic item", and how they might interact with your campaign by analogy to the dire bear.
If the Iron Man suit makes John Thug stronger, why won't it make Regdar stronger?
The thing about actual magic items is they work for anybody no matter what. They go on top of whatever you already have, whether your combat chops come from martial exploits, divine prayers, primal evocations, arcane spells, or psionic disciplines, they can make them better.
But you can't just pick up whatever the dire bear has and make it work for you. Admittedly it's easier to accept this when it doesn't seem like something you should be able to pick up, but the point you start from is this: the Iron Man suit doesn't work for Regdar any more than some butchered bear paw would.
In much the same way as "a dire bear" is not "a cave bear with a magic item", "John Thug in an Iron Man suit with a force trident" is not "a town guard with some magic items". You deploy them onto the field as a level-appropriate monster - in this case while regular John Thug is a human bandit, his power as a guard of Corruptica is inspired by the water archon shoal reaver (level 13 brute, MM3 p.18).
How did Regdar get strong enough to smash through an Iron Man suit with a great hammer? Training and experience. Like, literally getting experience points and training up in levels. How is the Iron Man suit able to dodge Regdar's great hammer strikes on occasion? Probably some weird wizard stuff, right?
If Regdar's expecting to directly loot and use the equipment, well, why should all of that weird wizard stuff line up with what Regdar already knows? Regdar tries to put on the suit and wield the trident, suddenly he's trying to dodge one way and the suit's trying to dodge the other, he angles in for a strike and the trident's autotracking nearly sends it flying out of his hand.
The Iron Man suit and force trident are "fake power", which created something on par with Regdar. They're not actually "magic items" that Regdar can pick up and use with his "real power".
But shouldn't Regdar eventually be able to make the Iron Man suit work for him?
Oh, you mean like how there's some guy in the base camp by the dire bear caves, holding up a sign that says:
I RECEIVE: five excellent quality dire bear haunches
YOU RECEIVE: level-appropriate magic item + minor quest XP
Maybe it's not exactly like that, maybe you've actually done it up proper with the wizened town elder being all "in these hills lairs a tremendous and destructive creature, the Beast of Bone Mountain. Be wary, brave adventures! But should you triumph over it, bring me its horrid maw and I will infuse your weapon with its power." and that's how Regdar got a +3 vicious great hammer (PHB p. 236) with a bear head on it.
Loot parcels don't have to be actual things you pick up and use immediately, they can be "waiting to be cashed in". So maybe Regdar spends some downtime bound and determined to make this suit obey him, he ropes some other party members into into it, and after a successful skill challenge it relents and he's got some +3 mountain plate armor. (PHB p. 231) Or some scrappy resistance sorceror is trying to jailbreak the force tridents and promising Regdar first dibs once they have enough samples to work from, and that's how Regdar got a paragon-tier set of breach bracers (Adventurer's Vault p. 116).
This is not exactly "bonus loot", in that it should be accounted for in the standard distribution of loot parcels per level that you're already giving your PCs. But if you attach some quest XP to the process (DMG p. 122) or make it a skill challenge, which awards XP on completion, it is kind of bonus loot in that the PCs get part of their XP to level from clever ideas and NPC connections, and thus got this parcel and its XP without fighting through an encounter.
But what if Mialee the level 13 eladrin wizard frog blasts the vent core and now the Iron Man suits are on our side?
What if Vadania the level 13 half-elf druid tames the dire bear? This is not a trick question, the answer starts on page 27 of the DMG2 and it's called "companion characters". Mostly the DMG2 material is about rolling your own so I'll give you the cheaty simple answer - drop an allied monster into an encounter, make someone responsible for dictating its actions, and it eats XP from every encounter equal to its own XP.
If you keep up projected XP awards, the extra bodies on the ally and enemy sides are going to stretch out combat, and monsters don't really have the kind of sustain that PCs do, having only one healing surge per tier to work with. So when Mialee starts laying down her l337 h4xx0r plans, probably the best time to have them actually pay off is for one or two big climactic battles, rather than making John Thug, Guard of Corruptica (sick hack edition) a permanent addition to the party.