I don't know if this has been posted or not...I personally haven't gotten a single Blockade yet, but one of my legionmates already had 2. Only he has gotten the blockades in our entire legion so far.

I have came up with more types of blockades that affect either production (which we already have), attack, defense, or specific types of planets (already have some).

Blockades that affect "Attack"
Different types of space entities that 'absorbs' attack, a NPC that has attack equal however much it 'absorbs' planetary attack (possible range: 20%-75% absorption).
(ex. 'absorb' 45% planetary attack = NPC matched attack + 45% of absorbed attack [Planet attack: 3000, Absorb: 45% (So -45% planetary attack), NPC Matched Attack: 2500, NPC Matched Attack + 1350 = 3850)
Possible Rewards: Artifact that increases planetary attack by x% for y hours (x and y are variables), or Planet Artifact: Size: 1, Production (choose 1): 1, Attack: 1000 (limit: 2 per planet) [suggestions]

Blockades that affect "Defense"
NPCs that reduce the effectiveness of defense structures by a %. Can affect 1-3 defense structures. If no defense structures are on the planet, no Defense Blockade. (possible range of effectiveness debuff of each defense structure: 25%-70%)
(ex. Planet Defense: 1500, Planet Defenses: 2x Planetary Shield, NPC Effect: -30% Effectiveness on 1 defense structure, 1 Planetary Shield: 750 Def. | 1 Planetary Shield (-30% effectiveness): 525 Def, Planetary Defense with Blockade: 1275)
Another example: [2x Planetary Shields (-70%): 225*2=450]
Possible Rewards: Artifact that increases planetary defense by x% for y hours -or- Planet Artifact: Size: 1, Production (choose 1): 1, Invasion Defense: +5% (limit: 2 per planet) [suggestions]

Blockades that affect "Specific Type Planets"
NPCs that blockade planets with these categories: Production priority, population priority, or type of planet.

Hopefully, if some of these are implemented, people will get different types of blockades, and occur a little bit more frequently for players that have large amounts of planets because there are more chances for the planet to be blockaded (but do keep it a rare enough so they do not occur too much or too many within a short time span)