Question: would it be a good idea to replace "when you would activate this" with a simpley keyword, "Activate"?
Example:
Balcony
$5 Action - Activation
+1 Card
+1 Action
Activate: discard a card.
Pros:
- Saves space by cutting out extra words.
- Can work without a separating line, thus opening up more designs space for those who don't like having cards with multiple separating lines. 
Cons:
- Makes it more difficult to do a "when you would activate" effect.
- Arguably inconsistent with other Dominion mechanics. For example, on-gain effects could have been written with a keyword too: "On-gain: gain a card costing less than this."
- Arguably looks clunky, especially on cards that have plenty of room for the extra text.
In my opinion, the cons outweigh the pros here. I'm for defining some terms in the rules as a shorthand, especially if they're used often. In would have been nice if "dig" were defined, for instance. "Activate" is already such a term, though, and I think having the rest written out makes the game more user-friendly (and, as you say, is consistent with other similar effects).
Side question -- what do you guys think of a "when you would activate this, do X instead" effect? From how I imagine it would work, this would basically force the card to be a permanent-duration while also giving you an effect that you would be able to play as many times as you want, so long as you have the actions to support it. The rules would have to specify that the action is still used up even though the card is not actually activated.
Would this be too crazy? IIRC, Donald said that Diadem was incredibly OP on some boards and so would simply not work as a regular supply card. A would-activate effect would be a lot like Diadem.
I would not necessarily rule out a card that you could activate as many times as you want, but for sure I am not doing one just to do it. It would have to be the best way to do some compelling thing, and even in that case, an Action that puts itself back into your hand is arguably better.
2 ideas now :
2. Couldn't we imagine a penalty for players that have inactivated cards in ply when the game ends ? For example -1VP for each of these cards. It could add problems of strategy in endgames.
That's pretty interesting. I like it.
I see no good reason to do this and lots of reasons not to. For one thing it's another rule in the rulebook, because it's definitely not being printed on each Activation card. It's also an easy rule to forget, which isn't a deal breaker, but also isn't great. If every card were like Boomtown, where the Activation effect is just a buyback ability, I could maybe see using this rule (and rebalancing the cards around it). But for other Activation cards you're already being penalized for not getting around to activating them. This rule would just exacerbate that penalty. Why?
I guess I understand the "increases endgame strategy" argument, but Dominion already strongly rewards endgame control, and I don't think it needs another push in that direction. If I did specifically want to favor endgame strategy, I would do it with some sort of bonus, as opposed to a penalty. A penalty is likely to make players not buy the card in the first place. But I don't think Activation cards specifically need an endgame bonus either.
Speaking of Boomtown and penalties, I'm thinking that perhaps "When you activate this, discard a card" should be replaced with "When you activate this, put a card from your hand on top of your deck." I think in general it might weaken the card, but overall I think it's a good change that adds another strategic element. I've learned to be wary of nonterminal cards that put cards back on your deck because it's really annoying to draw two cards, put one back, draw two cards, put one back, draw two cards, put one back, etc. But I think Boomtown would probably be fine because you're usually doing all your drawing first and then put cards back on your deck at the end of your turn, depending on how many cards and Actions you're willing to spend.
