This is a cool idea. As it is it reads a little like 'generic equipment'. I'm not sure that's a bad thing. Would all cards have slots or just specific types?
It would be much more tempting to play a cheap and weak 'browser' card, if it accepts more plugins than your typical card.
I think this would be stronger as a card mechanic than a game mechanic, since it would reward a certain strategy. If it becomes a game mechanic you have a wider base from which to reward strategies though. The difference is subtle, and my preference to it being a card mechanic is only barely ahead of it being a game mechanic. It ultimately depends on what you do with it.
A good friend of mine reminded me when working on another game to not get too caught up in all the 'awesome possibilities' while writing the base set. Although that is a really fun part of card design it can result in a hodgepodge of disconnected mechanics. What he suggested we should do instead is focus on the mechanics that we really wanted to form the base set, and a some great cards to highlight those. In the meantime you can still write down some of your cool ideas and reserve them for the next set of mechanics.
The reason I mentioned that is that prioritizing what mechanics and types of mechanics you want to highlight in the first set will help you figure out the role of plugins in your game.