I like it. Just some possible proud nails:
1) I'm assuming number of attacks will scale at 5, 11, 17, as normal, correct? I feel fairly strongly that attacks with a pact blade should be limited; 4 attacks at 1d10+5 is one thing, 4 attacks with a GWM'ed +2 Greatsword treads too closely to Fighter territory for my liking. (I'm assuming range, reach, and other properties of the weapon means that various weapon feats and magic properties would be usable.)
IMO, either the levelup fighter can handle another class being able to make 4 melee weapon attacks at high level if they choose a specific feature that locks them into a playstyle, or it isn't really an upgrade on the PHB fighter. I would either make this work on the warlock and then figure out if something needs to change in the fighter as a result, or ditch the gish warlock entirely. The worst thing about the phb warlock is that it
says that it has an option for warrior-warlocks, but it actually doesn't.
A compromise I'd be willing to make, would be to have the pact of the blade not add ability mod to damage, and/or do a smaller damage die, with perhaps a chart where only two-handed weapons without reach do d10, and everything else ranges from d4 to d8.
But the fighter should have enough other stuff going on in levelup that feat combos aren't nearly as important to it's identity.
2) Eldritch Cannon. Not a big of fan of AoEs with saves as a multi-attack at-will. I feel similarly about Eldritch Spasm from the playtest document, I just don't like forcing multiple saves. Maybe a 30' range 1d8 ranged attack with a 10' push instead (taking the place of Repelling Blast?)
If I could actually get EC onto a survey, and it got shot down, I'd just axe it entirely. No sense in trying to make it something else, IMO. Maybe make it an invocation upgrade to any ranged EB. speaking of which*
I'm trying to decide what sort of attack pattern is best for a bladelock gish; I'm thinking a mix of pact blade attacks and attacks with Eldritch Ray or Eldritch Scythe or Eldritch Whip.
IMO anything like that is too complicated for no meaningful gain. The attack pattern should either be the same as an Eldritch Blast/Ray/Spear blaster, or the same as a warrior class with Extra Attack. You have an attack that you make, and you can make it that many times as an action.
No, in a good way.
I like! It could be temp hp, advantage on certain rolls, or something like that.
I'd just run it like Summon Beast and the like. HP, AC, damage, etc increase with spell level if you cast it with a spell slot, and those buffs last for x time based on spell slot, before the creature reverts back to it's normal stats.
That's either going to be just like the standard EB reskinned, or too powerful because of its very long range.
Wait, what very long range? The range is either longbow range, which is only greater with a feat, or EB range. It's a force arrow longbow that uses your spellcasting modifier and can benefit from feats and the like. As a class feature, that's far from overpowered.
*So, I'm surprised I'm not seeing any "spend a spell slot as part of using Eldritch Blast to do XYZ, with numbers based on spell power" invocations and other features. I'm surprised actually that such features aren't in the PHB either. Like, heres an at-will ability with EB, and if you spend a spell slot you can increase the number of feat you move the target by ft per spell slot level, or spend a spell slot and deal EB damage to every creature within 5ft of the initial target, increasing the range by spell level, etc.