Just to be clear on this, I haven't opened Plantin and changed a few points. I've made a new font from scratch for which proportions were influenced by Plantin and kerning has been imported from Plantin (for the time being - I will definitely change it because it doesn't fit my design as well as 'homemade' kerning would). So I'm not trying to 'get away with' creating a new font quickly by modifying an existing one. I will freely admit that spacing is not my strong point and to this end, I have looked at how other fonts space glyphs as a starting point. There's obviously a lot more tweaking to be done on this one. In the end, I dare say it will bear little resemblance to Plantin. What do you think? Is it looking derivative? That is 100% not my intention. I give credit to designers who make revivals but that's not my thing. I think back to art lessons in school, where we would paint copies of old masters' works. The teachers weren't trying to create a bunch of forgers, but to make us identify their techniques and learn from them. That is what I'm trying to do here, but if appropriating similar spacing from pre-existing fonts is frowned upon, I'll simply keep the outlines and space them myself, for better or for worse. I hate spacing so much I might even consider iKern. Has anybody used this service? What did you think?

Foxtail terminals!

Foxtail terminals it is then - speaking of which, are the R and K tails too bushy?

I've added pdf sample to the original post. I've removed all kerning and changed the spacing to my own, so now all that's left of Plantin is the proportions, and even they have changed somewhat. No curves, points, side-bearings etc. are now in any way 'borrowed'. I'm doing it the long way, and hopefully learning more that way!
As I said, there's no kerning for now so it should give a clearer picture of where I'm going wrong with my spacing.
Also added numerals since last posts and thinned out the bushy tails of K and R.

This is becoming rather delicious! The numerals are especially nice, and I love the 4. It may need a larger counter (you could thin the stem a little perhaps).

A few other observations:

/b/ is floating above the baseline. ('blind' in first para.) Or maybe it just needs more colour bottom left?

Tail of /j/ is crashing into the baseline serifs ('enjoy' in the third para).

/i/ and /r/ look dark.

/s/ is a little too curly for my taste — I'd open out the terminals a bit more and let the counters breathe.

1 might benefit from a wider head.

6 looks a little squashed.

/R/ is causing spacing problems with its outstretched leg, in the third para. I like the shape so perhaps it could just be rotated clockwise to bring its foot in slightly?

The name suits the design, nice.

That right hand side-bearing on r is too small isn't it? Also I'm not a fan of the top serifs of T, and therefore E F and Z. Think I'm going to change it to be more like the top of 7. Updates shortly.

I personally preferred the curvy serifs on the E and F. The T looked weird for sure, but I think that's more about the openness than the shape. The serifs on Z need to be larger to balance the diagonal.

BTW remember to take my suggestions with plenty of salt and sugar, it's your design and I'm no pro. :)

I kind of liked the curly serifs on E and F but I think the new ones are better because they give the crossbar more room. I've decided on the above compromise - basically took out one anchor point and the serifs are now half way between before and after. Much happier with the T - it's starting to look more like a T and less like a palm tree!
BTW remember to take my suggestions with plenty of salt and sugar, it's your design and I'm no pro. :)
Clearly you are a pro! But seriously - I'm not going to go through it changing everything people suggest. Mostly it's a case in the critique section of people pointing things out which I'd notice myself if I was critiquing someone else's font, but when you stare at the same thing for too long you get blind to it.
The connected italics idea was just a thought really because my first stabs at drawing the italic look pretty rubbish. Trying to find a way of getting the 'foxiness' of the upright into it, but I've had a go at connected italics and this is definitely not the way to go with it.

It's been a while - finally got some time to come back to this. Still not braved an italic or bold yet, but have made a start on basic ligatures and the highly important foxtailed hederas

æsc time. 4 options ~
1. most similar to Plantin
2. less teardroppy a
3. standard double storey
4. Rowland form rears its ugly head again

spacing and kerning not fixed for any of them!

here they are in text - spacing is terrible for all of them, so try to ignore that

From the Millennial Oldstyle post where this cropped up, I don't think there are many occasions where ae and oe appear much in the same language, so ambiguities leading to confusing the two are rare. I'm leaning to #2. #1 just looks a little strange to me - almost like a swash small cap R

Well, you don't read or write it daily, do you? Of these #2 is the only half-decent option. This is how a you learn to write a lowercase æ in school.

How about this one? Moved the centre 'stem' to the left a bit to balance the a and e portions better

I'm working on the bold at the moment. Hoping to have it finished by the end of January. It will only be regular, italic and bold. Maybe semibold too but no bold italic. I'll post updates soon.

I saw that just the other day Hrant. Love the ffi ligature

I've added another pdf to initial post which shows the three styles together. I struggled to get enough weight into the bold /s/ but looking at it now I think I may have gone too far! Anything else glaring? I think I could do with narrowing a few of the italic uppercase characters.

I think in the italic they are fractionally too dark (especially some of the swashes) but I don't see it in the regular or bold. I'm going to make the italic uppercase slightly narrower anyway - at the moment they're pretty much slanted romans so too wide I think.

I'm pretty happy with weight of uppercase in regular and bold - there's a few that need lightening a touch (W especially) but all in all I think they're fine on test prints.

The last ae was the one I settled on.

Proviso: I'm not a type designer, or any kind of designer. It would be reasonable to ignore my comments. I'll say it, and then you can ignore it:

I like the updated T, but still would prefer crisper corners. Just seems un-T-like, even distracting maybe. I want the bar to feel like a bar and not a canoe. The upper corners on the E and F are more pointed--why not make the T's corners at least as sharp as them, maybe even more so?

This reminds me of a Plantin-ish font I saw in scan of a very old book. Actually it was attempted to be ID'd here, but no one could find an exact match. Now there's a closer one! (haha!)

Vulpa looks nice, and that promotional tableau looks professional to me (as an amateur).

But I'm sorry to say that it was the link to FF Pitu that really blew me away. Those |f|s! swoon