The Rogue Trader in the group I run has recently started to get an unhealthy interest in implants. Should I let him go wild, or is there some kind of system to limit this kind of thing? I haven't found anything, which is why I ask. I could easily see my group turn half cyborg if there's nothing mechanical there to stop them.

If you're really opposed to implants, you can always give negatives to fellowship rolls if you want. -10 every 5 or 10 implants will get the point across. This, of course, would be a bonus with the Adeptus Mechanicus.

Another route is just to not make them available, or make the currying of favor to get a hold of them an endevour. This is a good route if it's something decently rare.

I'm not opposed to implants at all, as I find them intriguing. I'm just a little worried that they're going to end up encouraging the group to metagame and min-max their way to becoming half machines. It's probably not going to be an issue.

If there are no set rules for this kind of thing, I might just hit them with a few corruption points if they go overboard.

That's not what corruption points are for - corruption points are for interactions with the warp, daemons, psychic powers, etc.

You could always do the same as with mechadendrites - you can only have as many implants as your Toughness Bonus. That could work.

I'm rather partial to the Path of the Omnissiah: http://darktrader.wikidot.com/character-rules#toc15

Check out the augmenticist in Into the Storm, this is probably the direction the RT is headed. Limiting implants is kind of silly. They can't stack armour past an extra 3 points from best quality subskin and maybe a 2 point bionic heart (if u let that stack) so the only real probelm is how bizarre the fellow wants to look. Implants of best quality can look very real and or impressive. Poor quality implants suck both visually and physically. Make it clear that this will impose fellowship tests, like -5 per poor implant and he'll go for quality over quantity - unless the implants aren't internal - in which case I impose a recovery time penalty of in game time to heal after the procedure.

Do you also punish them for buying Inferno Pistols and Heavy Weapons? Honestly, this is 40k. Part of the setting is the option to buy implants. Part of it is that some will go overboard. If your players want that, then what reason could you possibly have to object against it? There are worse things if they want to go min-maxing.

Badlapje said:

And when a player has amassed enough implants for it to be an issue, the character is most likely so insane and thoroughly corrupted that a few metal parts won't affect anything.

You might start to consider having some kind of RP effects for implants if a character makes it past rank 6 or so without being batshit insane and corrupted.

Machinator Arrays impose loss of some of the Fellowship characteristic so thats a good start.

Why would implants be a source of corruption? I'm not talking about something bought from hereteks or xenos, but honest to Omnissiah Approved-by-Mars implants? This isn't Cyberpunk with an arbitrary balancing mechanic built in.

There are no rule or cannon reasons for penalties for having a lot of implants. In fact as has already been mentioned there is a alternate careen path for someone who wants to do just that.

Your best bet if you want to limit it is the implantation and recovery rules in the Inquisitors Handbook (modified to taste of course). The players will of course usually have the best stuff, facilities and doctors but it will still put them out of the action for awhile. This will put a realistic limitation on it in that the players will have to devote time to it. Their player will be sidelined for a while as they recover and or learn how to use their new implants. Sure they don't have to learn how to use black bone bracing but to install it think for a minute what you have to do, flay the person down to the bone and then put him back together so a decent amount of recovery time is justified. In other cases like say a MIU, sure the recovery time would be a lot less but they then have to learn how to use it, feel free to withhold bonuses until you feel they have.