i have a few decks that could use some help.

So are these for FNMs?  Casual?  How competitive is the environment they're looking to play in?  Are you staring down tier 1 net-decks or home-brews?

Could you give us some details on each deck?  What are their primary strategies?  What are the win-conditions of each one?  What would the ideal starting hand look like?  How would they play out?

What sort of budget are you looking at for improving them?

Okay, so I guess I'll break this down in as organized of a fashion as possible.

First, if you're building an established deck like Maze's End, there really isn't any point in trying to make it your own beyond a few cards.  I get the aversion to net-decking, but as elpablo has said, and I have as well, there really is no point in trying to "remake" something that is already made.  Perhaps you can tweak it a few ways to fit your local meta more, but on average, if it's enough of a deck to be net-decked, someone has spent a LOT more time thinking about it than you - so why not use their work to your advantage?

Second, the previous point is not meant to discourage creativity or to say "just go net-deck lulz;" rather, don't be afraid to go outside of the box (milling for instance), but keep in mind that those strategies aren't very good and will likely end in a lot of defeats to the more established decks.  As much of a bummer as it might be, the established builds are established because they're good. 

Now, onto the specific decks:

1) Gate Deck - As I said above, there really isn't much point in remaking Maze's End.  Run with the more established build OR if you're really wanting something unorthodox, try to win via Azor's Elocutors and Sphere of Safety - think Heliod, God of the Sun (note his cleric tokens are enchantment creatures... so it becomes harder and harder for them to attack into you while your side keeps getting more and more stacked.  I can see where something like that or trying to figure out a way to abuse Isperia, Supreme Judge would be more amusing - pair him with Bident of Thassa and Riot Control/Aetherize - force them to attack, draw cards, return stuff to their hand, attack, draw more cards... enjoy.

2) Boros Deck - I tinkered with the idea of a token & Purphoros build.  The issue is that you need to be REALLY controlling.  Sadly the combo is extra slow and even once you get it online, it takes a while to kill them (at 2, then 4, then 6, then 8 that's 4 full turns of the combo to kill them - so that's what, a turn 9 kill?).  It might be worth a go, but you'll likely want to drop essentially every creature short of the Reckoners for more burn.  Think Warleader's Helix, Chained to the Rocks, and LOTS of that sort of stuff.  If you really want to have fun with it, why not run Young Pyromancer with the removal?  In fact, here's the build I tried (and ultimately scrapped), perhaps you can have some success tinkering with it - http://deckbox.org/sets/472508

3) U/B Mill - Typically milling is just a bad idea because it makes your job three-times harder than normal from the start... BUT... if you're set on going that route, we actually have some decent stuff at our disposal these days to accommodate.  Here's the list I had pondered for a while... http://deckbox.org/sets/481277.  The key here is that your list, as it currently sits, has some glaring holes - namely, not enough removal.  Blue/Black lack a solid sweeper we have to make do with lesser options like Cyclonic Rift.  This is fine if you get to seven mana, but against a more aggressive deck, you'll never last that long.  So you REALLY need to be sitting on tons of removal.  Far // Away is a GREAT card here, as it can bounce, force them to sack a guy, or, better yet, do both - potentially giving you a two-for-one in a lot of scenarios.  Liliana of the Dark Realms and Crypt Ghast are neat and all, but they clog the four-spot and likely won't last long enough to do the sort of ramping you think they will provide for Mind Grind.  Similarly, Desecration Demon just does not fit what you're aiming to do.  Additionally, what exactly was going to give you devotion for Nykthos?  Drop all of that mess and slide in some card draw and added removal.  Think Opportunity and Divination, think more Devour Flesh (who cares what their life total is!) and things of that nature.  If you're in a pinch, 1-2x Traumatize could be REALLY big for you.

Again, these will likely lose a lot... but if you get them to go off, well, it can be worth it.  I hope some of this mess helps.