There are now so many legendaries, and enough that are in the "hard to find" category, that I'd like to see more events with these "hard to find". Off the top of my head, that's:
1. Mew
2. Any Johto starter
3. Celebi - We got this through the Bank, however.
4. Either Hoenn starter not already distributed (Torchic was at release)
5. Rayquaza - I'm iffy on including this one; the Jade Orb, I think, can no longer be obtained without hacking or without having both HG and SS and two systems available to trade them locally. Otherwise, with Wifi gone, you must have Emerald to get the only other one available legitimately (Lv.70) as you'd have to trade on Ruby/Sapphire using hardware that is hard to find now - and again, two systems that can play those games.
6. Jirachi - I bet we get it if the Hoenn remakes come to reality this gen.
7. Deoxys - Various forms perhaps.
8. Any Sinnoh starter
9. Froslass - At Lv.1 with a special moveset, just because I think it would be cool. I have a Lv.1 Froslass from a trade, obviously evolved by stone upon birth.
10. Heatran - It can be found in Reversal Mountain, yes, but I think giving it a special powerful Fire move would be nice.
11. Regigigas - I haven't done it, but I think you can get one in Unova? The ones in Gen. IV are becoming more rare except by clones.
12. Manaphy - Because the Pokémon Ranger Egg was an event and it's becoming rare again, I think. Yes, you can hack the Egg into your game, but why not have other legitimate ones?
13. Darkrai - Cresselia is found on a bridge in Unova, so no need for it, but Darkrai is not found.
14. Shaymin - I think there could be a flower garden story behind this one on Route 4 somewhere since it seems to be the type of area it would like.
15. Arceus - Because you never have enough Arceus.
16. Any pick of Unova legendaries, though there were a LOT of events, so a break would be nice from them.
17. Unova starters
18. Any standard Pokémon that is hard to obtain in Gen. V and VI. Rattata and Meowth are examples though I'm cloning and importing a few of Yancy's Meowth and others once I have them all. Again, include a special move.
Note that any event starters should have special moves to make them good events. Otherwise, we can just import starters. I'm starting to do that now.