I took it upon myself to create a viable Mordekaiser build for Dominion since a lot of people seem to struggle with him in that format. It's not perfect but it definitely works, here goes:

Quints: Flat Health
Marks: Magic Penetration
Seals: Armor per level
Glyphs: Magic Resist per level

Summoner spells: Ghost + Ignite

Masteries: 24/6/0
Link to Masteries: http://leaguecraft.com/masteries/1330440011300030103300000000000000000000000000000?

Role: Solo bot, pusher. Mordekaiser, as ever, has excelled at demolishing minion waves in a mere couple of seconds. (He can do well in other parts of the map, I just start bot because the build is fairly item dependent. This gives Morde a chance to farm up without dying too much early on)

Skill order: I think of this as standard, but perhaps there's some variation. At the start, I put 1 point into Q, 2 into E and 1 point into W at level 4. Max E first, then W, then Q with R whenever possible. Maximum shield generation and durability with scaling damage to late game.

Item choices:
My starting item if my teammates allow me to solo bottom is a Hextech Revolver and 5 health potions. From here it's fairly loose and adaptable but I generally start off going for a Hextech Gunblade and Sorcerer Shoes. Spell vamp is a very good choice for him but I feel as though during skirmishes on Dominion he takes a bit too much damage for the rate at which he gains/loses shield. Life steal supplements that and the Gunblade also gives us a gap-closer other than Ghost. Between those two items I consider my core, the build can go 2 ways.

Option 1, doing well: Tri-Force is my standby for increasing burst during a 1v1. Q damage is increased versus 1 target and Tri-Force adds just enough kick to have your opponents on their knees in short order. It also gives us extra damage for our life steal and more damage for our shield generation. As always, a great chasing item with the additional movement speed and slowing-proc.

After that killing is no longer an issue for Morde with his amazing longevity in combat. All he needs now are some key defensive items which should be chosen based on the largest threats to you. Force of Nature and Abyssal Scepter for casters who are fed, Thornmail and Sunfire cape are excellent choices for AD carries. You can always do a mix, go for Aegis of the Legion, Guardian, and Sunfire cape.

Option 2, if you're getting owned: Go straight away for defensive items. If you're getting beaten up by a tanky DPS or hard carry, go for Thornmail first, and then consider finishing your Gunblade. If you're still having problems, add in a Sunfire cape. Against a caster, Force of nature and Odin's Veil should allow you to hold your own until help arrives from other teammates. A good combination for a mixture is Odin's Veil + Thornmail. In a situation where you can't assault their bottom lane, your best alternative is to defend your own bottom point. The longer you can hold out, the better.

If you get to a point where you're surviving battles, it's time to grab a Tri-force and/or Rylai's Crystal Scepter. I find the Hybrid weapons are extremely useful once you obtain a Gunblade because they increase the effectiveness of both Life steal and Spell vamp.

General strategy: By pushing the bottom point and applying enough pressure they cannot just have one person bottom, you create a situation where your teammates are now fighting a 4v3 battle a good majority of the time. Even if you die in the attempt, capturing that top point, and recapturing your bottom point in the worst case scenario still puts you on top of the game. If you can maintain that cycle and force 2-3 enemy champs to defend bottom, map control for your team becomes a cake-walk.

Good hunting

IMO Morde in Dominion is pretty amazing, maybe even sleeper OP.
I haven't lost a game myself yet with Morde

Ah. I'm sure people are successful, this is just a guideline for anyone who lost an interest in Morde since the recent nerfs. I was one of those people, but after winning every game played as Morde since I began this build, I'm starting to become a believer.

Come bot with your Morde. My Urgot/Cassi is waiting.

Eh, he kind of works in rare matchups bottom lane. He is absolutely horrible up top though. I run an extremely heavy CDR rune set up on him, allowing him to get to 40% CDR with just CDR boots. Kind of feels like he used to when he can spam hard enough to keep that shield up. Loses horribly to certain popular bottom laners of course.

Usually go something like Revolver, CDR boots, WotA, Rylai's, Void Staff, and then situational items as required. Early game is rough, shove the lane and run back for some potions if need be. Better your tower loses some health than dying and losing the lane. Buy a chainvest/negatron early in the build if needed.

I suppose you take the pots early on anticipating a lot of punishment, and not enough shield generation?

It's very uncommon to start with 5 healthpots, but i think if you can mitigate enough damage(morde shield) you can just make it work.

I'll ask one of my teammates(used to play morde a lot) to try your build, and see how he likes it

Morde is not a very good anything on Dominion.

-20% heal aura hurts.

He has low mobility.

He has no escape and is overy vulnerable to any gap closer's ganks bot ( akali, xin, etc can jus jump on u and gg )

If you build tanky = no enuff dmg
if you build glass cannon = u die

However, I have seen Mordes do decent with a Tanky AP-ish build if they switch to top mid to late game and your team knows how to focus the target you ult down. turning a 3v3 fight into a 4v2 is pretty OP.

i've tried morde once on dominion. the other team countered with four ignites. i was sad morde.

Starting to lose a few rounds here and there though, it's normally due to my teammates not being able to create enough pressure top. If your team is getting pushed back to your mid, that creates at least 1-2 very bored enemies who can now gank you bot without fear of losing much map control. It's extremely important for a Morde's teammates to keep that upper map busy, even if they aren't in control of that point. That's been my downfall a couple games. I've been pitted against some Yoricks, Heims, Nasus, and Brands and some of those games I've won, even though I lost a few 1v1s. In the end it was pretty back and forth even if I needed help recapping from my team. Got in plenty of kills and more often than not it results in a high personal score due to the amount of assaulting and recapping.

I love morde, ive loved him for awhile, and i play him in normals and ranked and win.
HOWEVER-
ive never had much luck with him in dominion. He just isn't a burst dmg champ.
and before you say "omg noob he's fine in dominion"-
there are so many champs that are OP as hell in dominion that can ruin you with insane CC and burst damage, i don't find it worth it to play morde and lose just because of the champ i pick. Dominion=so much less skill and planning than SR. morde is a skill and planning champ. (For those of you who say NO SPAM SIPHON AND WIN- please go play him and tell me that a no CC no gap closer no escape champ is easy.)

tl,dr

to me, Morde isn't good for dominion. there are better tanky APs. (<3 rumble)