That would've helped you a lot I think.Basically I see a few different things in your post.
Your players want to be villains
This one is basically up to you... do you want to run a campaign where the main cast is the bad guys? If you don't, and I can perfectly understand why you wouldn't, then just put your foot down. No, you cannot be the villain. Let someone else DM that game if they really want it. I wouldn't enjoy running that kind of game either.
Your players want to make their own devices
How comfortable are you with homebrewing? You can design and reflavor a lot, and get quite far. It can also be a lot of fun for players to be able to make their own things and then use them. Not to mention that you can get quite far with just plot
Your players want to play high powered characters
That's what levels are for. As levels grow, they'll get more powerful. If you want to up the powerlevel, just give them +10 levels through some kind of cosmic event and go over the top. Remember also that power is mostly narrative and just a bit of stats.
Make their quests bigger, their monsters nastier and their surroundings weaker and you get quite far.
And also try to figure out what you want. You're a player to, you should also have fun. If you're not comfortable with high level play yet, your players will just have to wait.
If you want to have a go at giving the players what they want, just start it up with a workshop on what the players want the adventure to be about.
You cannot challenge them with working to foil an evil plot, and a world full of compliant villagers that they get to roll over is very dull. So to start on the right foot, this campaign needs a point to exist - a valid enemy for supervillains with tanks, a reason why the villains are working togther, some kind of shared goal.
Once you have figured out something that binds the story together, the rest should be easier since you have something to focus on.
I would suggest, unless one of the players is already presenting some homebrew game mechanics for the "tanks", that such things are kept as fluff and don't actually affect D&D encounters, at least for the first few games. Sure they can roll up to the dungeon in their tanks (ignoring a lot of the wilderness as they get down to business), and weaponry can be re-fluffed to match the feel they want. Any more than that is asking you to do a lot of work upfront for something that might only hold the group's attention for a few sessions.
First and foremost we have the idea of your players wanting to be villains. I say let them do it, however make them starting villains...If they want to be evil overlords they have to start at the bottom as thieves and renegades and the like. They don't just get to walk into a town and start burning it down without any repercussions, and speaking of repercussions...they're the bad guys right? Thin about that for a second, in modern times if a terrorist group threatens a country we mobilize an entire nations military because of it. Why should their hijinks be met with anything less? but instead of F-22's the soldiers have dragons.
Next we have the problem of item creation, or in this case extreme weapon creation. There's a couple of loopholes to exploit here. First does any of the characters have the engineering or technical crafting skills needed to make something like that? If no, then the answer is no. If they DO have the ability character wise to do such a thing the next thing would be to say, "Ok your character want's to do this...you are your character...create some REALISTIC IN GAME STATS and then let me see them."
Also remember that if this was created, it's mainly an experimental technology and could be prone to breaking down quite Frequently.
As far as the last thing goes, my group always want's to create new characters so we've never had that issue before. Sorry about that.
-Hope this will help
Side note, thanks Cha-ika for the idea of anything to radical being experimental technology, this allows me to make them work a bit more before they can have fully operational tanks (not to mention drive a few adventures as they seek any existing knowledge that could help!).
The only successful evil campaign I played in, was a more mature game where the players understood that even though we were evil, we still had goals to accomplish, so we had to show self restraint of our murderous tendencies. And eventually the DM tricked us into saving the world, by simply keeping our own self interests in line with the good of everyone. But it was made easier because there were plenty of times we lied, cheated, stole and murdered our way out of trouble or made things easier for ourselves.
Another point to bring up with evil campaigns is to keep the players from being anti-party. Don't let the excuse of good roleplaying affect the party dynamics. Just because a player is playing an evil character doesn't give them an excuse to be a back stabbing, cheating, liar to the rest of the actual players. Even if it breaks continuity of the evilness of the players it should be understood that the players should still work together, and it just detracts from the fun of the game for everyone if the players are back stabbing and stealing from each other
Just my experience with evil campaigns. Unless everyone is on the same page and has a reason to be evil, then the campaign has a high chance of devolution and failure. Happy DMing.
I'm giving mine an option to live in the ruins of their world or trail blaze a new one. basically a big bunch of rocks start falling from the sky destroying kingdoms randomly and Survivors led by a Powerful Wizard open a gate to a new world...if they make it to the Gate in time they go to the new world and are forced to fight it out with primordials. If they dont they fight it out for scraps with the biggest and baddest foes.
I am just saying if being evil is just a means of taking over a country, you can do it under any alignment and claim the act completely within your moral code. Starting wars is the same thing. How many time through history have wars been started and fought by the righteous in the name of their god, against others who are the righteous of their faith?
I look forward to the synopsis of the player exploits, and happy DMing.
so sad)So far, they are acting relatively tame compared to what i expected.
One thing to use as a protagonist would be an uprising of "good" citizens (or at least a different evil) to cause them difficulty. Check out the novel "The Black Company" for a good example of a world where there are no good guys, only shades of grey with each one making the other look like the worse side of the conflict.
I've already started the ball rolling on having another group of "Heroes" to stand against them as they attempt to garner reputation within the city. I've thrown together a Reputation System loosely based off of the Victory Point system in Heroes of Battle so that they can track their Fame, and Infmay, throughout the campaign. Quests and Accomplishments that are more noticable grant more Reputation, and I even designed a few Feats that increase or decrease Reputation gains as well as some rules about dressing in a certain way to maximize how likely you are to be recognized. These "Heroes" are going to great lengths to increase their own reputation while harming the team's, as well as physically opposing the team in the future.
Sorry for the long stretch of silence there, one of our players dropped out and we were finding a replacement. The team has decided that the best way to make a name for themselves is to start up a mercenary guild, but as of yet have not named it.
A nearby town was having troubles with a cabal of necromancers that were raising the dead from the scene of a massive battle from about a hundred years ago. The undead that they were raising were unnaturally powerful, enhanced even more so by necromantic spells, so the team decided that outright warfare was not an option. Instead, they did a bit of research into the magic that the necromancers were using and discovered a device that would amplify the effects of the animate dead spell, but they could only be controlled if you had said device.
The team decided to play both the townspeople and the necromancers, saving the town to get a better reputation while extorting the necromancers for their aid. They stole the device (a very hair raising task, i assure you) and then sold it back to the necromancers in exchange for their promise to leave the area for 50 years (a pact that was overseen by a demon, thanks to the team's mage). They then helped the necromancers to find a new area to use in exchange for their promise of aid when the time comes.
Now they are traveling to a desert in the east in search of the Tomb of Ash'ai, an ancient wizard said to have created many items thought to be beyond mortal abilities (Minor artifacts and such). They are searching for his Grimoire in the hopes of crafting a set of magical masks/helms that will allow them to assume a second identity while worn in order to perform their more sinister actions without harming their reputations.