Things that the techie should be able to do:These are effects a techie can pull off. Things the GM shouldn't say no to, and the basic tools of a techie's arsenal.
A gadgeteer with inventor! should be able to fiddle with all sorts of options. These tools aren't so much 'better' than existing equipment than 'exactly what we need for the job'. This is helpful when dealing with weird and specific monster weaknesses.
A gadgeteer isn't just for building new stuff. He's also for building restricted equipment of of scraps. The electric pump connected to the gas can and garden hose isn't as good as a flame thrower for damage, but you don't need fancy fuel, can be quickly dissembled to made it legal to the authorities, its cheap, and it can be thrown together quickly.
A primary use of weird science is to counter the supernatural. The 'psychotronics' section of MH1 is all about ways that technology that neutralizes psi's, prevents mind control, can hurt ghosts, and so forth. These generally have a catch: either the protection is limited, or you have to hit the target with a weapon of some sort. Wide area complete power denial (like the null-field generator specified not to be available to PC's by default from MH5) is inappropriate. Countering or hampering specific abilities is another matter though (This field causes extreme pain for anyone teleporting into it). Most of this will use psychotronics and/or weird science.
What Techies shouldn't be able to do:Increase Total Power: A techie should not be able to increase the total utility of an invention -- at least not in a mainstream field. He can change what it does, combine two new technologies, or swap out one attribute for another, but a techie shouldn't be able to just plain increase the damage of a flame thrower -- though he may be able to come up with a concoction that works particularly well on demonic ectoplasm.
Don't Replicate Another's Power: A techie should not be able to render another PC redundant via tech. This is niche protection 101. The class most at risk here is the psi. If the techie wants to replicate a full psionic power, tell him no. There is wiggle room on this one though: controlled conditions and sufficient equipment limitations can waive this. For example, a hand held device that replicates seekersense (detect things you're touching a piece of) would be inappropriate. But a 40 pound machine that noisily destroys a DNA sample over the course of a minute and then points to the average location of the target over that minute is fine.
Don't Nullify Widespread Major Defenses: A techie should not be able to nullify the primary defense of a monster. In particular, he shouldn't be able to build a 'ectoplasmic disruptor' that makes the resistance of spirit beings to bullets irrelevant. Once again, sufficiently limited equipment should be able to achieve this.
Equipment Limitations:Gadgeteers should be able to accomplish more if their equipment is sufficiently limited. Or put another way, if a gadgeteer tries to accomplish something particularly powerful, his equipment is going to have some major requirements.
Deployment Required: Superscience tech has to be used in a certain way in order to be effective. It'd be nice to have a field you just set up and it damaged all ghosts in a 50 yard radius. While a witch can do that, the techie has to point and shoot his ghost blaster. A demon banishing field may require the demon to be hit from three directions (which no angle greater than 180 degrees). Or it may require setting up a circle of wire around the demon and then running power through it for a good 10 seconds before anything happens. An ectoplasm disruptor may require contact with its prongs, or be a liquid that must be sprayed on (and will run out), as opposed to a field projector that just requires power.
Side Effects: These should be aimed at preventing gadget spam. Headaches is a good one. Damage from prolonged use. Has a signature that gives you away to enemies. Costs FP. Nausea. Weird science is weird, and the most powerful gadgets tend to have major downsides to using them. Afflictions are a gold mine here.
Fiddly: inventions from a techie are fiddly -- they have a good chance of breaking down just by using them. Malfunction ratings should be low, and criticals should definitely break things. The afore mentioned teleporter's agony field may have a 10% chance of overloading each time it effects a target.
Awkward Shape: Classic Gadgets look funny. They can be big, bulky, fragile, and attract attention. These are major limitations in using one.
Things To Do with your TechieSo what specifically can I do with my techie?
Robots: Techies can build robots and drones with ease. These are generally best at surveillance and investigation, and will have a fairly high turn over rate. They are fragile, but the point is that it wasn't a human that got smashed. arming a robot is possible but illegal and not too effective. If you take this options you may need to talk your GM into letting you buy pilot (remote drone).
Counter the Supernatural : Counter the specific supernatural abilities of your foes. He can predict the future? I'll build a reality destabilizing suit! he can teleport in? I'll build a device that causes him pain (stunning and distracting him) when he comes in here. The demon has fire breath? time to fireproof the teams' armor! We're up against a witch? Lets see if I can build a device that detects magic use. If the GM doesn't want to allow an option, settle for penalizing the target when he uses a power, or merely impede the power.
Traps: Remember the tales of fiendish DM's from that other game who set up elaborate death traps? You have the skill to make this happen, and high tech equipment to work with. You still need to get the monster to walk into the trap, but that's a problem for the experiment or inhuman to worry about.
Holdout and Smuggling: The techie is king of getting things under the radar. You need to get past security? The techie can design a gun of all plastic parts and assemble it on the other side. It won't last long, but you needed it.
Signature Weapon: Techie's are big into modifying stuff -- and a weapon that a techie carries around long enough could have some unique capabilities. This is something you need to discuss with your GM to get approval for. You have a favorite uber-custom weapon. This weapon is an unreproducible prototype. You can modify it to hurt just about anything and do all sorts of things with it, including inventions that wouldn't be allowed elsewhere. As a downside, these inventions only apply to that piece of equipment. Buy it as signature gear worth 50,000$ or more. Its a good 5 points, but the expanded capabilities can be worth it. Many (if not most) players will want weapon bond with it as well. My personal version is a psychotronic projector with lots of different frequencies. Its big and bulky, and switching between modes is a ready action and a electronic operation (psychotronics) roll. Perhaps you can't build a generic anti-teleporation field. But if you use your precious generator, you can pull it off. This is particularly appropriate for Techie's with gunslinger.