I personally find items that expire to be distasteful and would prefer non-expiring trinket or weapon slots. Rather than costing 150/250 per month, I'd find it much more preferable if there were an option that cost, say, 5000/8500 and were permanent, despite that only being a cheaper option after approximately 33 months of continuous play.
(Also as an aside, a purchasable expanded ME slot might not be a bad idea either; for an up-front investment, a casual player could stock up extra energy for playing on the weekends.)
I'm pretty sure you're not going to get an expanded ME tank, useful as it would be. As it is, crafting forces most players to use CE in one way or another at 3*, and I'm betting that's by design. Right now 100 energy acts as a tidy barrier that a few aspects of the game are balanced around.
Permanent slots sound good, but I have to admit I think you're overpricing them. Really, something that wouldn't pay off until after a year, or maybe two, would probably be good enough. 2 & 3/4 years seems a bit excessive. But this is quibbling about the price, which is kind of secondary to the idea.
Permanent versions of temporary upgrades are often nice, even with the necessarily increased pricing.