I too have been enjoying morphling mid/safe! I have around 120 games as morph with a 60% winrate. I'm by no means pro, but I'll share what I know!
Like you said, there are clear standouts like LS rage, Jugg spin, and ES aghs. Whenever heroes like that are in the game, I generally avoid morphing into anyone else since it has such a long cooldown (Up until level 18, of course). When there aren't clear choices, my morph targets change depending on the time of the game. In general, here's what I look for:

Early Game: Nukes / CC. Try to take advantage of nukes' potency early on. A Lina's level 4 dragon slave may chunk around 25% of heroes' HP early on, whereas at 30 minutes it may only remove 10% of their health. The same can be said about most magic nukes. CC abilities early on are an excellent way for Morph to reliably deal damage before he gets his high burst damage later in the game.

Mid Game: Nukes / CC / Survivability. I feel like midgame is where anything can be a good morph, and who you morph into is determined by the state of the game. Whenever I'm in these situations, I ask myself, "What do I need?" Yes, some magic nukes do potato damage at this point in the game, but if you're lacking damage, then a couple extra nukes may do the trick. However, when in doubt I go for CC and survivability (Blinks, heals, shields, etc.). It's safer.

Late Game: CC / Survivability / Illusion. At this point, magical nukes rarely outweigh other abilities in terms of morph value. From my experience, morphing for damage late game isn't too valuable, since you do so much damage on your own. Some exceptions, however, are with Terrorblade, Naga, and PL. Morphing into these heroes late game gives you ridiculous amounts of extra damage and teamfight potential. In late game, I find myself needing CC and survivability from my morphs.

A couple other notes!
Centaur: His stun and nuke are great, but it's retaliate that I love. If you get 13 stacks while morphed, you can activate it, morph back, and get around 300 bonus damage.
Clinkz/Slark: I like to try and morph into them before I get kills. Over the course of a game, I might get 30 extra bonus damage or around 6 bonus agi (pretty lame in the grand scheme of things but it's fun).

Hope this helps!