Not a problem with diacritics. But I wish languages used them consistently, so that e.g. o/ö or e/ę relate in the same or at least in a similar way in all languages that use them.
For phonemic spelling systems, it wouldn't even be hard. Start with the baseline of aeiou for the basic vowel triangle. Then let's say that umlaut changes the vowel's "default" backness without changing roundness, ogonek lowers the vowel, tilde makes it nasal, and breve makes it a glide. This gives us aąäãăeęëẽĕiįïĩĭoǫöõŏuųüũŭ, plus all combinations of these diacritics when needed (which should be very rare). Add doubling to indicate vowel length, and I can't think of any European language that cannot be consistently expressed this way; American English should only need aąäeëiįouų.
For phonemic spelling systems, it wouldn't even be hard. Start with the baseline of aeiou for the basic vowel triangle. Then let's say that umlaut changes the vowel's "default" backness without changing roundness, ogonek lowers the vowel, tilde makes it nasal, and breve makes it a glide. This gives us aąäãăeęëẽĕiįïĩĭoǫöõŏuųüũŭ, plus all combinations of these diacritics when needed (which should be very rare). Add doubling to indicate vowel length, and I can't think of any European language that cannot be consistently expressed this way; American English should only need aąäeëiįouų.